Well, I am going to put my arse on the line and say" If your amp has given you problems before, and it's still giving you problems, well, then, connect the dots....."

Using my (inferior) powers of deduction, perhaps the power supply isnt producing a nice clean power signal (yes it produces dc power but I wouldnt hold a stick against it if it is passing ac line noise into the dc stream).
I would pickup some line filters and install them before the amp, on the +12vdc and the ground connector leading to the amp. Plus if the amp is not properly shielded (most are not, primarily the expensive ones are), then perhaps the psu is inducting ac noise directly into the amp, if the line filters do not fix the noise issue, then a nice lead plate between the psu and the amp would be worthwhile (not really portable then is it!) Or maybe those little ferrite donuts that sit around mouse/keyboard cords might help if they straddled your +12vdc and ground connectors?

Or just get another "known good" cheapo amp?

I suspect that these few suggestions might help you, I have been tinkering with the idea of doing this, but I think I would use an old Compaq 4/66 chassis (nice and slim, about vcr width) to mount a docking sled into, use the psu, but mount rca connectors onto the back, rather than an amp.

Let me know if these work.
Rounding up those ferrite donuts might be tricky, the only keyboards I have seen them on are the new MS keyboards with the built in usb connectors. But if you cn fine an old(er) kb with the donuts on them, and you are handy with a knife, I am sure you can coax those donuts out into the sunlight!

Have fun, and let us know if it works!

Smoker_Man
#080000449 MkII - 36Gb Blue