the 6505 + is the equivalent to the 5150 II - the difference is (1) input and each channel has separate tone controls - where as a 5150/6505 have (2) inputs (a normal and a high gain) and the channels have to share the tone controls. I have a regular 6505.
Just to be clear, are we talking about the stock Marshall 1960a/b Jcm900 series
yes, the one I have came with GT-75s, however even in the stock 800/900 series cabs you will occasionally see variations of speakers. Its more common in the 800 cabs and it depends on the year. I guess that's why there are certain model years that the tone elite like to rave about. I know the ones with the rola branded 65s are one of them.
It sound like you're playing much heavier rock stuff than I do though....I mainly need a huge variety of stuff to cover radio pop (think John Mayer, Owl City, Bruno Mars), Country pop, and church music.
I mostly play what I call technical punk. It has metal influenced leads but is written very different, and its generally not nearly as high gain as metal. I put an example up of a rockabillish track just to show its not all higher gain and that I was really working on clean tones too. there is quite a few bands that come in that are geared toward more ambient clean or even pop punk with a lot of clean to dirty and I need to go from fender type cleans to high gain.