I've never heard a v-4b, but I'd like to. Better not to though, I have too many amps as it is, and I'd probably want one. "But this one sounds different!" as an excuse only holds up for so long

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My Mesa isn't working yet, but now I know where to take it. The SVT2Pro I had would have been a new model, it was bought new last year and hadn't been in the store terribly long. The big reason I've stuck with the Mesa has been sentimental attachment, and knowing I couldn't in good conscience sell an amp that wasn't working right except for dirt cheap "as-is."
I'd say the Ampeg was more flexible tonally. Not that the Mesa's a slouch what with all the pull switch pots and switchable graphic, plus the 2 different preamp voicings. The Ampeg did a better scooped tone, the selectable midrange is a feature I like on any amp. Clean, it had ... I dunno, sort of a "smokier" tone than the Mesa. Capable of some real sweet overdrive, crunchy but smooth. When the Mesa breaks up, it's harsh and nasty. Mesa has more headroom, seems to stay pretty clean at just about any volume shy of earthquake. The Ampeg seemed a little groggier, less articulate--but that's not a bad thing if you want a big monstrous tone. The Ampeg I had was louder than my Mesa, but my Mesa may have been ill at the time I had both.
Mesa had more cab options, 2-4-8 ohm jacks where the Ampeg switched from 2 to 4. Don't remember all the features of the Ampeg, never used a DI on either amp or an effects loop on the Ampeg. Heard the Mesa DI was poor. One thing I loved on the Ampeg was the light on the back by the bias adjustment knob for user bias adjustment made easy.
I did love its tone. It was certainly close enough to "that SVT sound" for me (the Mesa couldn't touch that tone) but capable of more. Some of the reasons it went and the Mesa stayed:
I needed money for better cabs. The Mesa was old, thought it might be needing work, and the 2Pro was brand new, so it would get a better price.
Attachment. My brother was the 1st owner of the Mesa, it had changed hands between us a couple times but neither of us ever wanted to see it leave the family.
Fate. The same time I was preparing to sell off a tube amp for some cabs, another Ebayer was looking for a tube amp and had some great cabs available.
If it were possible, I'd still have all my big tube heads. It's hard for me to part with something with tubes in it. Except the Aguilar DB359. Not that I'm slamming that one, it was great at what it did, just what it did wasn't my style.