The problem is that most really good preamps are $3000 and up.

The PS Audio TRIO P-200, however, would sound lots better than what you have and is only $1200 or so; you could use that with the power amp section of your MF until you can afford a really good power amp to go with it. The P-200 offers fabulous sound for the money; the trade-off is that it is minimalist in some ways; no tone controls, no balance control, and only 3 inputs. If you can live with that, however, it is a bargain.

The next step up would be something like an Audio Research LS-17 or the BAT VK-31x; either of those would be very very nice-sounding indeed! I would go to one of them if you are OK with the $3000 range for a preamp.

Bryston is the way to go for a new power amp eventually, I think, unless you are a big fan of tubes (which I am not...too many reliability troubles with the tubes, plus limited bass...). I just bought the Bryston 3B-SST2, and it is to die for; amazingly transparent and quiet, with bass so realistic it puts most others on the back burner.

The most traditional upgrade path has always been to us an integrated as a preamp and add an external amp, but that was mostly because very few integrated amps had a unitary gain input. One way to think of it would be that if you use it as a preamp the power amp section will still be active and creating electrical noise thus reducing resolution. One the other hand I've always thought that all things being equal (and they rarely are in audio) having more power is always a good thing. But the speakers that you are using are supposed to be pretty efficient. I'm sure that it would really boil down to which was the relative weak link in the Musical Fidelity, the preamp section or the amp section. A good course of action would be to take the MF into a local dealer and try it both ways to see which gives the best return on "investment." If you don't have a local dealer pick a path and go with it realizing that neither is ideal and is a temporary situation on the path to separates.

What is it you find lacking in your present setup? Detail, volume, warmth, balance (tone wise)? Detail, tone, are generally preamp issues. Volume, warmth, usually are amp issues.
If you can define what you want, it can help you make your decision. I have bought new gear for no other reason other than I wanted to.
Go to a good dealer and listen to lots of gear, although sadly, this is hard to do nowdays.