Majors are nice amps, not really so tempermental as they are just removed from the abilities of the average hobbyist "tech." They run their tubes in ultralinear configuration, which means that the negative feedback loop must be carefully tuned and their output tubes MUST have very tough screen grids. Basically, they're a Marshall preamp stuck in front of a psuedo-hifi power section for the guitar/lead model and a PA/bass version, but they're not really all that different. The sound is very mid-focused. On the scale of relativity, with the Fender Showman being the "deepest," followed by the Sunn 2000S with round lows and more punch, leading to the Ampeg SVT being punchier still and low-mid focused, the Major is the most "brash" with more upper mids and less bottom and not much punch.
There's nothing out there like them, but there was also plenty of room to improve the design. Marshall themselves tried with the very limited runs of JMC800 2001 and 2000 amps in the early 80's, with the 2001 being called the "Bass Major" and the 2000 was called just an ungodly overpowered guitar amp.
I have a 4x15 slant-front cab that was part of the 2001 bass cabinet series. It looks like an oversized 4x12 and uses four Gauss 15" speakers, but is ported. There is a straight-front 4x15 that was meant to be the bottom of the "stack," with the cab I have being on top of it, but I've never encountered that model.
FWIW, if you want Jack Bruce's Cream tone, just get an EB-0 and blow up the tone cap in it, no kidding. I played a 63 EB-0 with a blown cap and it sounded like Jack Bruce in EVERY amp I played it through, and all of them were s/s, mostly cheapie practice models. The distortion came from the bass; I've never heard any amp, including a Major, replicate that sound, even with equally old EB-0's that were intact.
