In my experience with Monitor Audio -- mostly the RS-6 -- the best sound came from using tubes, which gave a tremendous soundstage and the sense of ease and space between instruments that I've rarely experienced with any other speaker/amp combo.
The RX and RS series are very efficient speakers which allows the use of lower power tube amps. Another option I like was using a McCormack DNA-1 -- very satisfying but different from tubes, more of a clean sound with lots of inner detail, but still buttery smooth and effortless.
Heck, even an old B&K ST-140 sounded decent if you don't have a lot to spend on amperage. Not as much detail and soundstage but still very pleasant to the ear. JHMO.
--Tom in Sacramento