jeffd75 you might want to take a look at this blog post which goes into a lot of detail about the various trade-offs with the analog i/o.
i) Normally the analog inputs convert a voltage of 0v to 0 and a voltage of 4.096v to 1. When using the audio expander capelet, 0v is converted to -1, 2.048v is converted to 0 and 4.096v to 1.
ii) If you don't have anti-aliasing filters then any part of your signal (including noise) which is above the nyquist frequency (half the sample rate) gets folded back into the lower frequency part of your signal, which is far from desirable especially when doing feedback control. It is almost always worth putting an anti-aliasing filter when reading sensitive signals, even if its only a simple passive rc-filter. The audio expander capelet adds active second-order anti-aliasing filters to the analog inputs for you.
iii) 16 bit accuracy is easily enough for this sort of thing in my experience. It can be difficult to get your noise floor down to below 1 least-significant-bit of a 16 bit signal with Bela's analog inputs, which is something like 62.5uV.
iv) Maybe @giuliomoro can recommend you a preamp, but I think if you're using the audio expander capelet you should be able to connect the mic straight to Bela's input?
That said, I wonder if using a microphone as the input to your feedback system is the best idea. Have you investigated using optical reflectance sensors or something similar to sense the movements of the gong directly?
I can't wait to hear this thing by the way, with the size of the gong and the power of that transducer, it's going to be impressive!