Autodexr - As everyone above describes, you're in the land of "the hardest thing to do right" when you're talking about reproduction of complex orchestral passages. Everything contributes (or takes away) from that aspect of recording reproduction.
Small-ish speakers can do very well with complex passages--it just depends on the speaker. I use Harbeth Monitor 30's, which, being monitor speakers, are designed to handle as much as you can throw at them...but it depends on what you throw it with. I was getting really annoyed at Stravinsky's own recording of the Firebird on CD--the loud parts were edgy and grungy sounding. I got an Lp version, and found significant improvement. Sadly, I think the CD medium is (often) part of the problem, b/c I'm using some pretty good players (Wadia, Linn, EAR). I agree with Elizabeth that the amp is a significant factor, too. That having been said, one odd thing that can happen is that on lesser equipment, the loud parts sound okay--you just know something is missing. For example, I was listening at a buddy's house to Bartok's Miraculous Mandarin with a $100 Sony CD changer, through Vandersteen 3A sigs, which I would have thought would have been awful, but it sounded pretty darn good...though I knew we were missing a lot of detail.
So, the short answer is, what you're hearing is normal. The really hard part is, how to "fix" it. I think Rushton's suggestion that you pull the speakers out from the wall (if they aren't already) may be a good start.