If you're looking for more power and clean headroom out of a V4B, the hot ticket has always been to have the amp retrofitted with 6550s - however, you can probably expect a change in your tone as well.
You'll get a slight increase in headroom, but the 6550's require twice the bias of 6L6GC's/7027A's and will probably idle far too hot unless the bias circuit is modified but it's not worth the hassle. If you need more volume that badly, get a bigger amp, another V4 used in conjuction, or use more speakers. 6550's won't give the amp more than a 5-10 watt boost.
Actually, some later V-series amps carried a label on the back panel which read "7027A/6550 - Use one type only". I think there was a factory hop-up kit offered...
Not so on everything but the tube chart, and that's only half right. Ampeg did use 7027's and 7027A's and specify them, but it was because they got them at a discount from RCA, who was trying to use Ampeg's reputation for power to help sell them after the hifi market ignored them. A 7027 is just a 6L6GB / 5881 with extra screen grid connections, which are unused in Ampegs, and the 7027A is just a 6L6GC / 7581 with extra grid connections. If those connections were used in ultralinear configuration, then those tubes could
potentially develop more power than their standard brethren in the right circuit, but there is no advantage to using them over a regular 6L6GC in Ampegs. The 6550 is a completely different tube, the next 'step up' in power in the US beam tetrode family of tubes: 6V6, 6L6, 6550.
Having said that, there's something about those 7027s that just flat out works in the V4B. Some have said that you can get similar results with a good NOS 6L6 type tube (like a 7581), but I have not had the chance to explore that.
I've got lots of both (and a couple of V4's). They're the
SAME tubes.
As far as my expert witness says, there's a huge difference between the V4 and the V4B, as they work in different areas soundwise.
There isn't. There are only minor parts value differences in the preamps and one is easily converted to the other. Their power sections are the same. The quality and output of the power tubes makes a much bigger difference in sound than the amp being a "B." Any differences your expert hears are far more likely attributable to the quality of the tubes in the individual amps than the model designation.
He's got at least four of the B:s, but won't let go of any of them. 
And the V4B:s I can't find on Ebay. Except for one in the south of France. To get them from USA is to expensive for a simple guy like me. 
If there ain't anyone with some economic ideas?
Get a 'regular' V4. They're more plentiful and not very popular as guitar amps, and hence, generally cheaper than V4B's. If you absolutely HAVE to have it to V4B specs, any decent tech can easily convert the preamp.
