Thanks for the linky, some cool stuff, I know someone who converts old valve radios to dab and mp3 players, his source for bits is a guarded secret.
You could try 2 seperate supplies just as a test, in reality however they ought to work off 1, try a large cap close up to the power supply input to the power amp, like a 4700uF, and put a 100nf disc ceramic accross it too.
Have you tried another speaker?, or maybe 2 speakers in series to lighten the load just as a test.