I forgot you had a vintage Super, and didn't know you had the Nomad! Glad I asked.One is the Super Reverb, the old 1964 one I’ve had forever. Great sound on its own or with pedals in front. No MV, no loop, and a 2 ohm output, so it’s definitely its own thing. Years ago I bought a Weber Z-Matcher, so now I can run it into my other 8 or 16 ohm cabs, and it sounds great but also shows how much a SR relies on its cab to sound like the old, loud, mean amp we all know.
The other one is a Mesa Nomad 55, and it’s best described as a Super Super Reverb. Similar in configuration with 2x6L6GC power (you can also use EL34s with the flip of a switch) into 4x10 Jensen Alnico speakers, 55 watts (I always thought the Fender SR sounded more like 50+ watts than the rated 40), but with three independent channels each with a Gain and Master, then an overall Master and footswitchable boost. As Randy Smith did, take a Super Reverb, then add every function possible within that framework. Sometimes overdoing ruins a good concept, but in the case of the Nomads, it was one of his best “do it all” designs. I gigged Nomad 55s (the 12“ version combos and head/cab versions) for years before the Road King II came along and it’s a solid, roadworthy band-format gigging amp. We did everything from old Motown to Stone Temple Pilots and it covered them all.
Back on the 4x10” subject, my prediction is that you’ll find something similar to your humbucker/P90 comparison, ie as many similarities as differences. There’s a lot of middle ground with 12” open back cabs, but still always it’s own flavor. I haven’t played anything but the alnico speakers in one personally, so I’m waiting to see what a trained ear thinks of the GBs.
I wanted the 4x10, as you say, to get a different flavor; even with the Jensen alnico 2x12, it was different from my other 2x12s, but not ALL that different, so the info about P-90 vs Humbuckers is a good sign of things to come later today. Um...if I live to tell the tale!