Re: Your favorite boundary microphone?
Boundary or Boundery, I know what you're talking about. Some people just have too much time on their hands, lol. I usually don't use Boundary microphones inside bass drums, but find them useful as desktop discussion microphones. Being close to the table top lessens phase problems that might erupt using standard microphones on short stands.
Between the two microphones that you mention, the Shure Beta 91A has a much higher SPL rating than the Sennheiser. So if I would need a microphone inside the bass drum, that would be my choice. Under extreme pressure inside the bass drum, I'll always go for less distortion over tonal differences. The stated maximum sound pressure for the Sennheiser is only 124 decibels which is easily obtainable in most bass drum applications. In my opinion the e901 just needs a bit more headroom in that type of application. Otherwise I love Sennheiser microphones, 421s, 441s on and on.
BTW, there are a few other Boundary type microphones, but my money would go with the Shure.