Oh, i hate to step in this, but i'll give you guys my 2 cents worth on the audio hum.
Its the cabinet, or more specifically, the way the wiring is run inside the cab, combined with really aged capacitors. Dc voltage is dc voltage is dc voltage. A good switcher will put out power that's every bit as clean as a linear supply in most cases (or at least good enough to not hear an audio hum).
I've got a tron with one of Mark Spaeth's power adapters, and a spy hunter with the original power supply. On the spy hunter, the power supply pcb has been rebuilt a couple of years ago, but i haven't changed the big caps in the suitcase power supply. They both hum. One is about as bad as the other.
I also have a test bench that i built from scratch. It has a switching power supply and an original mcr dual amp pcb. When i first built it, it had a horrible horrible buzz in it. I finally replaced all of the audio signal wiring with sheilded wiring, and it is now quiet as a mouse. I've been tempted to rerun the audio signal wire in my tron with shielded wire physically seperate from the rest of the wiring, just to see if it helps the hum. I kinda think that the hum may be ac buzz picked up from somewhere in the harness itself. Either way, i know from personal experience it is possible to make an mcr amp quiet with a switcher, the key is (or was in my case) running good quality sheilded wire to keep any a/c bleed off the lines.
Now, having said all that, if i were building a jamma cab to satans hollow adapter, i'd just use an amp from a set of computer speakers driven off the +12v line. Before i built my new test bench, if i wanted to test sound, i just used a set of amplified computer speakers, and alligator clamps between the stereo jack and the pcb. It worked fine without any buzz.
