Marshall JMP Superbass II alternative?

The differences between a Super Lead and Super Bass are few, did you try the Plexi? I believe the only difference between them are four capacitor values and one resistor value.

I Googled it and came up with this:

The bass amp and the lead amp evolved out of the fender bassman circuit, and the earliest amps are almost the same except for the volume brite cap, which is used to tune the lead amp slightly briter.

The original bassman (jtm45) design is brilliant because it is both very bassy, very midrangy and brite at the same time ... the first pre amp tube is is biased to give a tremendous bass thump to the signal with its 220mfd bypass cap. The balance of the circuit actually tunes out lots of bass and the tone network centers both the bass and midrange controls on the amps midrange. Both Leo fender and Jim Marshall were intelligent enough to realize that their customers would run their amps Flat out, and realized that the amp needed to sound good on 10.

marshall super bass amps evolved in materials and parts as various venders and specs changed over the years, but remained with the original bassman like circuit with its 220 mfd (bassy ) bypass cap up front

On the lead amp, around 68 Marshall decided to split the 2 halves of the first tube and retune the brite input to a much more trebly setting. They changed the 220mfd cap to a .68 mfd (quite a difference) ... the bass channel retained its 220mfd bypass cap. Many lead amp users Jumper their bass and treble channels to mix some of the bassy sound back in ...
Other folks just go with the very brite and cutting lead channel.
Other folks have a tech go into their lead amp and tie the cathodes back
together with the old setup. Just as many folks have their tech take their super bass amp and spilt the cathodes and try and convert the amp to lead specs.

great examples of all these tones can be heard on recordings

as a general rule of thumb I would say that fender players prefer the lower volume fatness of the bass amps, while humbucker players would prefer the Lead amps. One could also divide the camps along blues tones and metal hard rock toned lines.

There has been an increasing interest in Super bass amps of late, when I was first pursuing them they were almost a giveaway and were often frequently modded /butchered. Many "lead" players turned up their noses at bass amps and then got sucked deep into the MV mod and attenuator game... The lead amps are tuned to sound full and singing flat out, and these were "Stadium amps" by design.
The bass amps were sleepers for a long time they sounded full and fat at all volumes and are easily pushed into distortion with common stomp boxes, while they are bassamps they are 100 watt "tweed Bassman" amps. The Fender bassman has a ledgendary lead tone stock, even though its a "bass amp"

from here: Vintage Amps Bulletin Board • View topic - Super Bass and Super Lead difference
 
FWIW, when I saw Lemmy/Motorhead in a club he was using two JCM-900 guitar heads thru two stacks...he had a bass sound I've not quite heard before or since...
 
If you're going after the 50 watt sound you also may want to try the 1987x. For the Superbass tone I'd also would try the "normal" channel with the Plexi or the 1987x.
 
I got a good enough sound with the Plexi Treble and a tone match. I don't really care too much about getting it exact. It sounds great as it is.

FWIW, when I saw Lemmy/Motorhead in a club he was using two JCM-900 guitar heads thru two stacks...he had a bass sound I've not quite heard before or since...
Sure that wasn't Phil's rig?
 
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