There are many paths to concentration. I have known students who doodle because it helps them focus on a lecture more effectively. Others like to listen to music while they study either to help them focus, or to drown out other noises. If my kavana is increased by the removal of distractions, may I put headphones on while I daven my shmoneh esrei? [related, but about learning]
Would it make a difference if
a. I had ear plugs in
b. I wore headphones but nothing came through them
c. the noise I listened to was ambient/environmental (rain, waves, white noise) as opposed to instrumental music (and is that any better/worse than music with lyrics)?
I know that one should be able to hear himself davening but even with headphones on, one can hear his own voice.
One answer I have heard repeatedly is that we should conduct ourselves in a way which would be appropriate were we talking to a head of state. If I wouldn't address the president wearing headphones, then I shouldn't when I daven. But I also wouldn't address the president with a shawl on my head, or wearing leather straps, or shuckling so that equivalence is not exact.
If the goal is to increase my kavana is there any limit to what I can and can't do (barring actions which explicitly violate other halachot) to increase my focus and concentration?