The secret with subs (and it appears it IS a secret, considering all the boomy farty noises I so often hear from other people's) is that they should not be as loud as many seem to think.
The other secret (!) is the crossover point/filter is often set way too high, so the sub is shouting out all these low-mid frequencies as well, which our ears can pinpoint. The result is a nasty muddy mess, which chav boy racers in their silly cars like, but to anyone who still has ears that work it is an awful noise.
With the exception of some bass drums, very low bass guitar, orchestral bass and some synths, musical instruments (and voices) are pitched above the level that a sub should be reproducing, and so you should almost never get very much coming out of a sub with music - only in movies should you really be aware of it, and even then it should not dominate, just add.
The exception is these systems like Bose Acoustimass etc where the satellite speakers are tweeters with a bit of mid, and the sub isn't so sub, as it has to make up for the lack of mid-bass in the system. Some people like them - and that's their choice - but for me they totally destroy any natural-sounding soundstage. They're ok to watch movies on, and casually listen to music, but if you're serious then IMHO they are pretty hopeless...
Just my few rubles... 