Re: 5150/6505
the 5150 is based upon the soldano SLO and the 6505 is just the 5150 with a new name. the 6505+ is the same but with a second tonestack. IMHO, the second tonestack sucks away tone.
The soldano SLO is an evolution of the JCM800 if I recall correctly.
So, what's marshall anyway? JTM45? JMP 2203? JCM800? All three marshall, all three archtype marshall tones, but all three are extremely different amps.