My vote goes for the block letter 5150 as well. I could only get the gain up to 3.. Not 3:00, actually 3 on the dial.
Anything after that was unusable.
I Put a Sovtek 12AX7WA in V1 to try to knock the gain down a bit and get some tone out of it.
Worked somewhat well but still, I always found that odd on those early amps. The latter signature models you could usually get the gain a bit past halfway.
Aside from that amp, I would say the Diezel VH4 has endless amounts of gain.