to get teh best our of an amp that big you need to run it a little louder than very very low volume.  it's to do with getting the speaker cones moving.  and sometimes pushing a speaker hard can sound good to the ear.  to get a 4x12 moving hard, you'll probably deafen yourself in a garage, or get arrested, or both.... stacks aren't the best thing for home practice, which is why 1x12 combo's are made too.

now that aside, there are things you can do, but what you will find is that you may need one set of presets for low volume and another set for higher jam/gig level work.  this is the same for all amps from all manufacturers, and it's more to do with how the human ear hears different frequencies (bass middle and treble) differently at different volume levels.  se here: http://en.wikipedia.org/wiki/Fletcher-Munson_curves

think about some home stereo's which have a loudness button, and it makes the music sound better when playing loud.  well that's to compensate for the FM curve effect.

to compensate on an amp, you can use an EQ pedal, and often people who gig would set their sounds up to work at gig level, and then use an EQ pedal to compensate for lower volumes, etc.

basically if you set your patches up for low volume, and then you want to tweak them for high volume, you'd need to increase the bass a couple of increments,  and treble by a little bit (less than the bass).

if you set up your patches at high level, then to tweak them for low level to the oposite.  lower bass a couple of points, and lower treble a touch.

right now onto basic patch editing. find out what amps BFMV use.  then look in your SV manual (or downlaod it here: http://line6.com/support/manuals/ ) and chapter 3 tells you what amp models the SV has.  choose a similar one, then turn the bass, mid, and treble to 12 o'clock, turn the reverb and effects off, then turn the channel volume to 12 o'clock.  next turn the noisegate and distortion boost on if you need them (it tells you how to in the manual in chapter 2).  then tweak the tone settings little by little until you're happy.  then add effects and verb afterwards.  also remember that the pickups on your guitar, the tone and volume setting of your guitar, and the age of your strings will all affect tone.  I find that after strings get older than 2 months they start to sound very dull, and can really make your whole tone suck, so change your strings if you haven't for a while.  you may find your tone comes back to life.

another good thing to learn how to do is the hidden feature that's mentioned in ch2 in the manual, and that's how to find out what settings a preset has.  this is good for mis jam session tweaking.... as remember that the knobs positions means nothing, as it's all stored in the preset... which is unlike a normal tube amp.

hope this helps

Rowbi