So, you've read through 158 pages of this thread (as I did), and you are on the cusp of buying a V-4B reissue (as I was). My advice is to just go and do it.
I've only been playing bass for just on a year (changed over from guitar) and I'm stoked to have found what I regard as the ultimate tone almost straight away. Bought a modest Fender Rumble 200 watt head and 115 V3 cabinet as my starting rig. Great tone. Just not loud enough, really, for our enthusiastic and rather heavy-handed drummer.
(Oh, I have a 2015 MIA P-Bass)
After about 4 months, last August I bought the V-4B head and another 115 cabinet and ran the head at 4 ohms. Great tone. Super happy and loud enough for my needs.
Couple of interesting things:
I took on-board everyone's advice and ran the V-4B with the master wide open. Ran the gain anywhere from about 9 to 11'oclock and that was loud enough and clean enough for my needs. Great, fat, round tone. Used a Bearfoot BBBD for grit and bit of drive - awesome and transparent drive that left the natural amp tone alone. How could things be any better ? Well.....
Firstly, recently I started backing off the master. Set the gain at noon and adjusted master to volume needs. Things just opened up. The EQ controls have a more dramatic effect on tone. It sounds more airy, lively, there's just the right amount of breakup that is touch sensitive. Don't need or want a drive pedal anymore. Sounded even more awesome. How could things be any better ? Well.....
I sprung for a 212AV cabinet a few weeks ago and turfed the 2 x 115V3 Rumble cabs. Holy f#$%balls. Whatever voodoo that the ampeg people have worked into this head/cab combo was immediately apparent.Played a couple of small open mics/gigs and random people were coming up complimenting how awesome the bass tone was and how it held the whole band sound together. Sounded phenomenal to me and obviously to others. Surely things cannot get better ?
Got to watch the band's bassist after us play through my rig with a Travis Bean (?) bass. Also ridiculously awesome.
Sorry for the long narrative. Thought this might help another fence-sitter (as I was).
Oh - and the volume question asked a million times ? Yup. In my loudest situations, with the master wide open, and the gain control at 11'oclock, it was just starting to break up but has still met/exceeded all my volume needs. With the gain at noon and the master around 11/noon as I run it now, it meets all my volume needs and has the greatest natural breakup that is so good, I don't WANT "clean".
I cannot imagine playing in a band/situation where I would need a 300w SVT. But that's just my personal circumstance.