I am currently running a Mark Levinson 390s directly (balanced ic's) into a pair of MC501's with superb results.
As for the 501, I have found this to be a wonderfully balanced amp - the bass control, midrange liquidity, top-end smoothness and soundstaging are utterly impressive for an amp in this price range (at any price really). IMHO, you would have to spend considerably more to realize any subjective "improvement". Of course, this is all dependant on sytem matching, but I would definitely try to audition a pair of 501's at home.
As for the 501, I have found this to be a wonderfully balanced amp - the bass control, midrange liquidity, top-end smoothness and soundstaging are utterly impressive for an amp in this price range (at any price really). IMHO, you would have to spend considerably more to realize any subjective "improvement". Of course, this is all dependant on sytem matching, but I would definitely try to audition a pair of 501's at home.