I alsolutely think that a valve preamp is your way to go and will give you the best results for the cash you have. The 5150 power section is an absolute monster and stays clean at insanely high volumes which makes it a perfect power amp candidate. Just plug a seperate preamp into the effects loop return and use the master volume.
You could pick up a Marshall JMP-1 on ebay for under £300 that will give you all the Marshall tones you could ever want. If you can find one, the Marshall 9001 is the rack preamp version of the Silver Jubilee series amps and they usually only go for around £150
I would also point out the Johnathon (Feline) has Engl and Soldano preamps on ebay at the minute for £299 a pop, both absolute bargains.
I just feel that buying a complete new amp in your situation is false economy and won't get you what you want. The power-amp section of the 5150 is as good as anything you will buy, so why not utilise it.
If you can stump up more cash, I would look at the new Rocktron ProphecyII preamp. I'm seriously impressed by it and will have one in a couple of years once there are deals around on them.
