Well that does seem like you should be getting enough bass to balance out the treble, but since you aren't, maybe the speakers are indeed the problem. The MA6500 has pretty powerful bass for its size so I don't think that's the issue. I know because I bought one from my neighbor. It's a great amp.
Maybe your space is Really large and needs more bass than what 4 German 9s can offer. It's hard to say without seeing and hearing your system but a subwoofer may be a solution.
Tonal balance is a relative effect. If you don't have enough bass, all you notice is the treble.
Otherwise, I would recommend the Marantz SA11S2 for your system. It has a warm, laid-back sound that might suit it really well.
