I love my 5150 212 combo. It's got the signature, not block...even though I don't think the combos ever had the block letters...Anyway, I love the 5150's distortion, it's very smooth compared to other amps. I also have a 5150II head and the matching 4x12, the difference, IMO, is that the II has a more modern and "aggressive" sound...but not *TOO* different from the original. It's there, just gotta pay attention. I too swapped the last 12ax7 for an at7, much better sound, to me anyway. I payed $450 for my original 5150 combo on eBay, new they were like $1200 at the time. I bought my 5150II half-stack for retail price...I forget how much. Yep, latah.
~006