Did you have sound before you removed the radio? Easiest way to test the HK amp is put the old radio back in and see what happens. It will be very difficult to test the HK amp otherwise, as you won't know if it is the MOST adapter or amp itself.
If you get sound with the old radio, it is one of 3 things: 1. New radio is not outputting any sound (get a home audio receiver and one speaker, bring to car, and hook up to one of the RCA outputs from the radio. If you get sound, you can rule this out. 2. MOST adapter not working. 3. MOST adapter not connected properly or is not getting power (test w voltmeter)
If you get no sound with the old radio, repair the system (including HK amp) until it works, then swap to the new radio.
I had sound up until the comand unit stopped working, this was several months ago as I just planned to go with this Avin unit when I was ready, so its hard to tell if the amp went bust along with the command unit.
I will test the outputs from the Avin tomorrow, everything works though, steering wheel control, mic, bluetooth, reverse camera etc. Pulling the amp is a bit of stress so I will check the output which will narrow things down to the interface and the amp.
Then I will read the interface to see if it is actually passing some power through it and hopefully this will give us one of two things that need to be changed.
It would be a pity though if its the interface or the head unit, Avin has very bad support response and hardly ever respond to emails, this is a shame because the quality of their products seems to be right up there.