Hello again gents

After some research I've found that the cab I've got with my 6505 head might not be as great as desirable to my style. I've been advised to try and change to a Marshall 12x4 vintage, possibly the 1960b, as opposed to the one I'm using now, Peavey 5150, specially in terms of brilliance and output. All opinions will be most appreciated. 

Thanks in advance 
jonalcover What kind of sound are you looking for? The 1960b seems kind of overpriced for me, it is just 4 $100 speakers in there at nearly $1k. I do not think it will be much louder, although methinks you would get more of a Marshally-vibe from the G12t's. The typical recommendation for the 6505 are Vintage 30's, you can find the 6505 4x12 cabs with V30's loaded for under $400.
The only thing wrong with the 5150 cab is the speakers, to be honest -- the construction is very good, however.  The Sheffield speakers in them are generally mediocre at best; they're not terrible, but they don't allow the amp to reach its full potential either.  The deal here is that you probably won't get much for it if you sell it, and you'll probably pay upwards of $400+ for a used Marshall cab.  Even more for Mesa cabs.  Generally, Celestion Vintage 30 speakers are the go-to speakers for 6505s.  But there are similar speakers by Eminence that are cheaper and IMO sound just as good, if not better in some aspects.  I got a used Crate 4 X 12 cab for practically nothing a long time ago; made $50 selling the speakers out of it, then put in some Eminence Governors and Swamp Thangs in an X-pattern.  The cab is an absolute beast now.

That said, depending on where you live, you might find a Mesa Recto cab for $400 used, and they're loaded with the "Mesa" version of Celestion Vintage 30s which are the best V 30s made.  Where I live, however, I can't touch a Mesa cab for less than $600, and that's for a cab that's been dragged behind a car for several miles, if you know what I'm saying.  Same goes for Marshall cabs.  In my case, it was cheaper to just buy the speakers I wanted, and the cab itself is very well-built like your Peavey, and exactly the same dimensions as a Marshall 1960A.
Will Lane thanks for reply Will. the sound I'm after is a bit hard to get with nowadays's gear. Massacre's from beyond, Death's human, Sepultura's arise, ... are guitar sounds I'm a suck for, and generally most of Scott Burns classics. It feels to me that this hissing end on the top of a heavy and low tone is the hell of a work to get. 
KailM thanks mate, I will do some research about the Sheffield. Bit odd to hear about some speakers with an english city name, oh well. I will have your list in consideration and collate prices of what I can find for a reasonable price.