Worth upgrading from MC611 to MC901?

FWIW-I run MC1000s and an MC2000 into N801s in a similar  configuration.  Adding the MC2000 was a definite improvement in my listening experience and I have no regrets.

I don't know what kind of an alternative you mean, but if I were in your position and could add tubes to your high end I would go for it.

Bill

I'm concerned about all the heat the mc901 would give off

I have not owned the 611's, but the 901's don't run as hot as you would think. The SS portion of my 901's never even run warm to touch. The tube section will radiate heat you can feel with your hand a foot away, but are far from anything I consider excessive. For their size and power they are very easy to live with.