mcbr1 -

It seems there could be several things going on.

1) speaker issue
2) source issue
3) amp can't drive the speakers well at high level w/ complex music (so an amp issue)

My only advice, since you seem to be on a tight budget, would be to demo anything you want to buy before selling what you currently have. Take time and don't rush this - because you could end up with something worse than what you have now.