Ian,

Reading the 2nd half of this post are some questions I didn't answer.

Our in-ear-monitors are simply very very very nice and efficient passive headphones, that are in-the-ear rather than over-the-ear. All frequency and gain adjustments occur before the monitors, as you suspected.

You should be able to get more gain into our IEMs with an external amp, with more frequency range efficient drivers in our IEMs, than with hearing aids. You will also be in control of all of the compression that occurs. Hearing aids have lots of compression algorithms that you may or may not be able to disable, that can be problematic at times for your purposes; even with the 'music' program that you might have available in the hearing aid.

-Scott