Rob, one way to go is to get the best power amp your money will buy, and put a passive "pre" in front of it. If your source output impedances are low enough, their output voltage high enough, your inter-connects short enough (more specifically, their total capacitance low enough), and the power amp’s input sensitivity and impedance high enough (the MR RM-10’s sensitivity is under a volt, it’s input impedance 100k Ohms, both making for an ideal candidate for a passive pre), you should be good to go.
No matter what amp you decide to go with, there is a great detailed description of what went into the design of the RM-10 on the Music Reference website. In reading it, you will get some insight into the choices an amplifier designer is faced with, things that don’t get talked about much: why a low impedance power supply is of benefit to a circuit, why "over-rated" parts are a real good idea (learned during Roger’s youth, when he started building amps in his family’s basement, his 3 years as a repair technician in a Virginia hi-fi shop, and as Chief Engineer at Beveridge), factors that go into the design and construction of a transformer (which has a profound effect on the sound of an amp), factors that effect tube life, not to mention what is involved in getting a tube to operate in it’s most linear range. Good stuff!
In a private email, you asked me about current solid state amps. About them I can be of no help, having only old ones (Electron Kinetics Eagle 2A, PS Audio 200C) myself. The only modern ss amp I find intriguing is the Sanders Magtech, which is way over your budget. What I can say is that given the Advent's impedance characteristics, and it's low sensitivity, I think you'll find a ss amp to be a better match than a tube one. Just one opinion. In the early issues of The Absolute Sound, Harry & co. were using a Phase Linear 700 on their Advents.