Fast, a good clean sub works equally well for both music and HT. SVS subs play low, loud and clean as their measurements show -- in the deep bass the group delay is on the high side. Find the most recent Stereophile and read Larry Greenhill's review of the PB13-Ultra.
Regarding a crossover (or bass management controller as it's sometimes called for multichannel systems)... As you'd expect, you'll get differring opinions as to whether or not you should use or need one. The fact is that woofers produce distortion as the frequency decreases or the SPL increases. Just look at the sub distortion measurements. For example, the REL T1
to make 30Hz at 90db produces 6% distortion, at 25Hz at 90dB it's 22% distortion; raise the level to 100dB and you get 14% at 30Hz and well it can't make it to 25Hz at that level.
Subs have 10", 12" or larger drivers (in fairly large boxes), what happens when a 6.5" driver (in a smallish box) is asked to produce bass? B&W says your 805's distortion is less than 1% from 100HZ on up at 90dB at 1 meter (not 2 meter as the subs are tested so this makes the values look better than they really are). What do you think the distortion is at 50Hz at 90dB? Maybe 5%, maybe more, maybe a lot more.
The idea behind using a crossover is to protect the main speakers from bass frequencies and route those frequencies to the subwoofer which is designed to handle them. The speaker system will produce much lower distortion levels this way. Purists will say that adding another piece of electronics to the chain only mucks up the signal. I agree that the crossover may add maybe 0.5% distortion to the signal, but compared to what the main speakers will do without a crossover it's a no brainer to me.
As a side benefit you gain some headroom in the amp by removing a portion of the audio band it has to handle. You are actively bi-amping with the amp in the subwoofer.
Hope this helps a little and good luck in picking a sub. You will need an SPL meter and bass test tones to calibrate it.