Depends on how heavy you want the sound... I've found most if not all Mesa amps have the ability to get you that rich harmonic distortion that you can't describe but hear in your head... the problem is that most of them are very hard to dial in if you've never used a Mesa before. I'm STILL finding new tones and I've owned a Mesa and used it extensively for 2+ years now.
That being said, definetly check out the manual when you get it and figure out how all the tone controls effect each other. Apologies if I'm telling you things you already know.
Some amps come to mind, depending on how much you'd like to spend.
First off, the Rectifier series will give you the 'biggest' sound with the sacrafice of ease of use getting a nice tight clip, they tend to sound extremely huge, especially through Mesa cabs; but some players find that they have trouble dialing in a nice harmonic crunch that's super tight and elastic. The cleans on these amps are also nothing to write home about in my opinion. Another thought, the contour mode of the F50 gets pretty close to recto territory but not totally.. this may be something to consider if you don't like that mode on your F50.. though still try one. The tones ARE there but it's definetly not as easy as say my next recommendation, the Mark IV
The MarkIV will give you insane high gain clarity and focus to your notes, very quick attack if desired. It's a great 'all purpose' Mesa in that it has solid clean tones, insane high gain and everything in between. The sacrafices with this amp are (if you consider it one) that it won't get as menacing as a Rectifier nor as huge sounding.
If you've got the cash, the Roadster and/or Road King will give you the most versatility and features.. however a RK might be overkill for your needs.
A Stilleto (depending on which series) will give you a pretty dead nuts Marshall killer in a box that can hit those tones at a much more reasonable volume than any actual vintage Marshall. The first channel is similar to a plexi and the second a 'hot rodded marshall' like a JCM800. If you're a marshall guy this amp might be just up your alley. The newer ones are much more aggressive so you may not like that. The Ace looks promising to me, it may be my next amp.
Finally the Lonestar series, which are a totally different beast.. it will nail the classic Fender tones with ease and add new levels of gain to those tones if you want to. The Lonestar Special's ability to operate in 5 watt mode is so cool, it sounds amazing.. however you won't get some of the heavier Mesa tones out of it, it's still got a ton of attitude to it and might be what you're after.
Go spend an afternoon with a few at a local shop where you can be in a private room if possible. Ask for the product manuals or better yet find out which models interest you and print the manuals off the Mesa website, not only do they have some 'instant gratification' type settings in their pages, they'll be invaluable to figuring out how the hell to refine these tone monsters.
