Preamp is not the only thing contributing to the sound and marshall had "100W" jmp amps with actual power ranging from 70watts to about 130-150(all with 4 el 34's) and they definetly sound very very different from each other.
The signal chain in soldano rectifier and, apart from having an extra stage with identical values as the previous, 5150 is not similar, but identical. Exactly the same copied, whatever you want to call it. They have different channel switching and that does change the schematic, but the signal chain is the same. That's probably the reason they sound similar.