Usually for amplification, I've used either a condenser or dynamic on a stand in front of the bell, or a clip on mic with an extension. I prefer the latter, but it sometimes reduces the bandwidth of the input, but not by much. It's just easiest since I don't have to rely on a venue or institution for mics - as much as possible I'd love to keep things self-contained (minus a PA). Like I had in the gear list, I have a ton of cables and pedals and everything to hook this all up to my own mixer.
I've found a couple of ways to effectively use contact mics. I can place them on the bell, which picks up most of the bell resonance from actual playing, but also is obviously great for any percussive bits on the bell (or perhaps bowing it with a bow or sheet metal or glass, etc). The other is to stick one, or hang one from the trigger, so that it becomes a more active sonic participant. Both of these I use in Katie's piece.
tube feedback (with capsule mic)
This is something I've been playing around with with improvising. So two options, the first is a little more blunt, but the second is more malleable. So if I have an output monitor relatively close to the bell of the instrument (say on a table or plinth beside it, or even on the floor), I can essentially wire up a microphone to generate feedback through the instrument. By taking the actual capsule mic out of the clip-on housing, it's small enough to be sealed into a mouthpiece using blu-tack. The fundamental of the instrument then affects the pitch and the overall spectra of the feedback, as it's fed through the physics of the tube. For the second option, which I've used a lot with a euphonium, is that I could take out the F slide and seal the microphone inside of the open valve slide. That way I could engage the feedback loop only when I press the trigger down. Since the slide is before the valve, I'd still have total control over pitch, although it may be a bit different without the bell flare. I find it to be more interesting harmonically, since the resulting spectra are way less harmonic and stable. This way I could still be playing normally, then depress the trigger and allow the feedback to kick in. The only hard thing is that it's quite loud, with very little dynamic control. But through an insert line, I could process it with whatever effects are possible.
These are obviously awesome, and could be affixed to anything. might be really nice on a wood plate affixed to the bell - this could maybe then trigger some of that feedback I mentioned previously. The smaller tectonics transducers can be affixed to the instrument, but sonically they don't do as much as I'd like - things sound better when they're attached to a mute or something. I'm happy to turn any mute (except the wooden ones) into some form of transducer mute & yeah, the transducer on wood plate sounds pretty awesome... I have all of the tech needed to assemble these. Since the instrument is metal, I could also conduct transducer feedback (probably via contact mic) through the actual instrument itself since it's resonant metal...