If you're trying to spot imperfections and differences in sounds, this is easier (and much cheaper!) with high-quality headphones (Beyerdynamic, Sennheiser...) than speakers, unless you're fortunate enough to do what high-end studios do and have very expensive monitors in an acoustically-treated room.
Fenton, your point about studios might be correct in ones churning out chart trash, but it's not in the ones I've used. Most critical mixing is done using high-quality monitors (e.g. ATC), the NS10s (or whatever) are used to check that it still sounds OK on lower-fi gear, for example where low bass notes will be inaudible. If you're then doing a demo track to be low-quaility-streamed on the web more compression might then be applied to get the average level up, but this isn't the version that would make it onto an album.
Nigel11, if you're so convinced that MP3s always sound crap and add all these non-harmonic tones, why not put your money where your mouth is and try the test I suggested? (uncompressed vs. 256k VBR MP3)