I don't know why, but it seems like almost every iPod speaker I hear here at CNET is a wretched-sounding thing. Most have screechy treble, lumpy bass, and vocals never sound remotely human.
As always, you get what you pay for, and the cheapest ones tend to be the worst offenders, but hey, they're cheap.
Some, going for upward of $300 are somewhat less horrible, but for three hundred bucks, you could actually buy a nice set of hi-fi speakers.
And since most iPod speakers are one-piece systems, they don't do stereo all that well. Sure, many incorporate some sort of processing to simulate stereo separation, but that usually messes up their already pitiful sound quality even more. With separate speakers, you can place them far enough apart to make stereo sound like stereo. Which stereo speakers, you ask?
I like Klipsch's little 2.1-channel iPod solution, the ProMedia 2.1 iPod/Computer Speaker system that goes for $150. It features a pair of two-way satellite speakers and a 6.5-inch powered subwoofer. Separate speakers means it does bona fide stereo, and the sub is big enough to generate real bass.
For $199 you could buy a pair of Audioengine A2 speakers and hook them up directly to your iPod. In my opinion, the A2 sounds at least as good as any single-box $400 iPod speaker I've heard. Granted, the $600 high-end iPod speakers make a lot more bass, but it's still on the thick, boomy, and bloated side of natural. And they're $600! For that kind of dough, you could buy a small stereo receiver and actual hi-fi speakers.
You can read myhere.
If that's more stuff than you want to deal with and you still want more bass than the A2 makes, go for its bigger sibling, the A5 ($349 per pair). And if those little guys still leave you hungry for more low-end oomph, go buy an actual subwoofer. Auudioengine's baby S8 ($349) sub will rock your world, and knock the socks off any $600 iPod speaker on the planet.
I'm slamming one-piece iPod speakers for their iffy sound quality, but if that's not a top priority for you, go ahead and splurge.