Is it permitted for Bnei Torah to go to a classical music concert?
There are a couple of issues that must be taken into consideration, whether music is permitted and "moshav leitzim":
Regarding the first issue see Shulchan Aruch Orach Chaim 561:3 which discusses the prohibition against listing to musical instruments in remembrance of the destruction of the Beis HaMikdosh. The Rema brings a number of qualifications to this, and the Mechaber himself notes that the custom is to be lenient regarding singing praises of Hashem while drinking wine. The Seridei Eish seems to understand the prohibition against musical instruments as being the main opinion all though he notes that in Germany this wasn't really on the radar, he nevertheless prohibits holding a secular concert in shul (Seridei Eish O.C. 16:20). Rav Moshe Feinstein (Igros Moshe O.C. 1:166) seems to favor being strict but notes a number of considerations for leniency, though not all would apply to secular music.
While the most obvious issue, perhaps, is the issue of instruments there is another issue. Concerts are highly problematic because of the prohibition of Moshav Leitzim, but ask your Rav:
It is clear that Moshav Leitzim is a seperate prohibition from bitul Torah (which is why it is called by a different name) but they are related. The Gemara in Avodah Zara 18b says that events and activities which are "moshav leitzim" bring one to bittul Torah. See also Rashi on Tehillim 1:1 and Igros Moshe Y.D. 4 simon 11.
Moshav Leitzim, we can see from the poskim, does not require that the entertainers engage in leitzanus per se. This can be illustrated by the well known piece from Pirkei Avos (3:3) about people sitting together without divrei Torah, it is called moshav leitzim without any qualification on what IS spoken about. Rebbinu Yonah, as I understand it, comments that their neglecting to do so makes a "mockery" of limud haTorah.
The application of this to contemporary events is not, basicly, a disputed ruling:
“And this is the ruling for one who goes to theaters and circuses which are places of amusement and pastime.” (Aruch HaShulchan 307:9)
See also the Magen Avraham 307:22 and Chayei Adam Hilchos T’fillah u’Brachos 63:17.
From these sources we see that the prohibition applies even to Jewish events (M.A.224:3, Be'er Heitev 224:2, M.B. 224:3). From this, and the Gemara in AZ 18b itself we can infer that these events a problematic even when there isn't avodah zara or violence.
Indeed we see this prohibition brought down by various contemporary authorities regarding to current entertainment events, see Igros Moshe O.C. 2:95, Shemiras Shabbos K'Hilchasa 16 footnote 25. While other activities including books and games can fall under the category of moshav leitzim, the dynamic at work with circus and theaters, and by extension concerts, seems to be one of passive entertainment in a group setting. This, even with my expansion, is not exhaustive in explaining the issues, but I think it is clear that one should not make decisions about what constitutes "kosher" relaxation based of of speculative "explanations" about what constitutes moshav leitzim. And while it has been noted that Rav Chaim Pincus Sheinberg shlita has given a lenient p'sak it is very difficult to reconcile it with the Poskim (Divrei Chachamim 5:42) nor is it clear that every form of contemporary entertainment would come under this heter, although presumably .
We should not make the mistake of ignoring the explicit rulings in halachah in favor of, at times dubious, stories about Rabbis doing this or that (especially when young):
איו למדין הלכה לא מפני למוד ולט מפי מעשה עד שיאמרו לו הלכה למעשה -בבא בתרא קל:ב
A warning which should also serve to remind us not to render decisions based on what we read here but rather to consult a Rav.
Any lenient sources are more than welcome but it has been my observation that individuals trying to present their own line of reasoning about this topic end of permitting that which the poskim have prohibited.
I know this should be posted as a comment on Yirmiyahu's answer but 1. there is not enough room and 2. I believe this requires real hora'ah for clarification:
I question the Feldheim translation of "s'hok" as amusement based on the fact that the "Batei Tiatros" (theaters) and "kikusaos" (circuses) were of a certain ilk. The ancient circus (and maybe the modern one as well) devaluated human life through risky and sometimes downright deadly performances. The ancient theater was essentialy a place of gathering to witness re-enactments of legends of pagan mythology which were full of immoral and degrading behavior. It is easy to see how they can fit into the category of "Moshav Leitzim". A classical music concert is in no way making light of anything important. Si'hok means laughter and in this context, laughter of leitzanus. I think this is an example of a question which requires real hora'ah from a poseik.
Yirmiyahu has presented (in my opinion) a conclusive case that Feldheim's translation of the Mishna Berurah is in keeping with the intent and usage of the original. He has also presented the Seridei Aish's opinion. Barry has presented room for other approaches to understanding Hazal based on Rishonim. We have heard, albeit not in writing, that Rav Scheinberg disagrees with the MB and SA. We do not know who asked him and in what context.
We need to investigate if there are any "contemporaries" of the MB or SA that argued (barring reports of Gedolim having season passes in their youth, since, yes, it could be they did so (gasp!) against halacha and then such reports could fall into the category of Lashon Horah on the dead without toeles, which is under a ban by the Rema).
If there are no contemporaries that argue we get into the (sticky) question of whether the MB is accepted as the Poseik Aharon on this matter.
Yirmiyahu: No offense taken. You brought valid and relevant sources, while I haven't.
The plain meaning of Moshav Leitzim is the company of scoffers, see Radak on Tehillim (1,1). The mishna in Avos you quote isn't meant to contradict this meaning, rather it is derived from further on in that chapter, where it states "Only in the Torah of Hashem is his desire". So the root of the issue in that Mishna is Bittul Torah. If the gathering isn't Bittul Torah, say because it provides a way to relax after a stressful week, and it doesn't stray into Leitzonus or other prohibitions, such as a classical music concert, it is not a Moshav Leitzim.
See also Avodah Zara (18b) and Tosefta (ch. 2) where theaters and circuses are described as a Moshav Leitzim, specifically becuase of the Bittul Torah involved. Once again, if the Bittul Torah aspect is addressed, there's no Moshav Leitzim.
Essentially, I am arguing against a position that "there is no such thing as kosher entertainment". There is Bittul Torah at one extreme, there is Limud HaTorah at the other extreme, and there are a range of activites that lie in between.
Another point which nobody has mentioned here yet:
Very often, classical music concerts also feature an operatic performance which often involves the prohibition of kol ishah. That would make it off limits for any Jewish male, even if not a ben Torah.
See http://mi.yodeya.com/questions/977/is-it-ok-to-watch-to-american-idol-during-the-omer for some relevant points.
Is one allowed to listen to non-jewish music altogether? According to R' Ovadia Yosef in Abia Omer one can. However, many say that since the non-Jew who wrote the music imparts his soul into the music, it shouldn't be listened to. (If you look up R' Yosef's tshuva, you will notice that he gives the hetter based on the fact that many great kabbalists listened to non-Jewish music. Perhaps one can answer that since they were so holy, they weren't affected, while normal people would be).
Based on anecdotal evidence, yes, one is permitted to attend not only a classical music concert, but also an opera with singing.
As noted in Hillel Goldberg's From Berlin to Jerusalem, Rabbis Yitzchak Hutner and Joseph Soloveitchik attended operas and concerts. In fact, according to Rabbi Rakeffet-Rothkoff, Rav Hutner even had a subscription to the opera in Berlin.
Rabbi Soloveitchik held that the prohibition of kol isha applied only in situations that would engender improper thoughts. Indeed, Rabbi Soloveitchik attended the opera, and considered it to be “advanced culture”, and that far from being prohibited (for containing kol isha), that its attendance was actually to be encouraged. (Source: http://www.jewishideas.org/articles/new-hearing-kol-ishah near the bottom)
Re "...reports of Gedolim having season passes in their youth, since, yes, it could be they did so (gasp!) against halacha and then such reports could fall into the category of Lashon Horah..."