MC transformers - what do they sound like?

All of the ones I've heard cause harm to the bass and reduce transparency.

I agree. I have had no luck with them. I think the j-fet before the tube gain stage ( as in the new Audio Research phono pre amps) is going to be the way to go.

You're going to get a some interesting responses to this question. The two sides will probably confuse you more than help you. All I know is that I have a Hagerman Trumpet MM phono stage and the designer himself confirmed for me that the MM version of the Trumpet combined with a quality MC step-up was a better configuration than buying the MC version of the Trumpet. In any event I think system synergy is more critical when using a MC step-up as you're adding another piece to the chain, but I have experienced great sound with my modest K&K Lundahl and haven't had any issues with bass.