Dayum, congrats! My Sunn 1200s is pretty much the same power section (4x 6550/KT88 ultralinear) but you got a middish preamp and I've got dark.
I can't turn mine up passed 2-3 at jam - you are a brave man.
The 1200s is going to be more similar in power to the Major than the Super Lead. Super Leads and Basses came with 4xEL34s, or during some period, 6550s in America, but the actual power section never changed, just biased for different tubes. The Sunns also had significantly larger transformers. My 2000s and 1000s are all much louder than the 100w Marshall I had, and also louder than my Hiwatt DR103. But the mids on the Marshalls certainly help them push through!
I think I keep the Sunn rig on about 3-4 during practice...but I'm kinda missing my Marshall now.