hmm I think I might chime in a bit here.. one of my first real tube amps was the good old 5150+evh5150III cab.. at that time I was struggling to find a good clean tone and overdrive..
so I started a research on which amp to get next and eventually i ended up owning a mark 4A..
The main reason for that was that I liked the idea of 3 channels+GEQ features and so on.. If I were to compare a 5150 and a mark 4 I'd say - you won't get that raw growly sound out of a mark 4 (maybe a MARKIII would be a better match for those kind of sounds) but the closest youcould get to a a 5150's lead channel is the rectifier I'd say
One of the main reasons I chose mark 4 over the recto was that I was not a huge fan of that boomy low end rectifier sound - even though it's usually on the player's fault if something sounds
cr@ppy..and the next thing is that each recto I ever played was hmmm stiff.. as if I had really had to struggle quite a bit to get a good sound out of it and i am not one of those uberfast lightpickers who use tons of gains you know.. thats where I knew that the recto would be great for me only if boosted properly, which would be a step back in ergonomy (extra pedals, cables, step dancing, bypass loops and stuff)
i found out that being a 7string player I am more into mids/upper mids and later after having spent a year and half with the mk4, I moved to a mark 5.. I have to say that the mk5 completely delivers it all because it's even a step further from the mark 4 who's got mutual EQ pots for channel 1 and 2... the mk5 channel 2 crunch is another great palette of sounds and I admit I spend quite a lot of time switching between ch2 and ch3 :)
it really sounds good even on low volumes when needed (unlike the mk4) another thing is that with those push/pull pots on the mk4, each time I dragged the head to a new place I had to reset all my settings cause some of the pots would eventually move, get pushed in or twisted while being carried over...
there's no such thing with the mk5 now and each time I carry it somewhere I just randomly check to see that all's ok and ready to be fired up :)
is there a difference between mk4 and mk5 lead channesl? yes it is, the sound a little different.. but it's usually the bedroom players who can tell th difference :) in a band context you won't mark a difference..