I've had a dozen or so MC240s over the years. Following are a list, by preference, of the preamps I used with mine:
Marantz 7c - IMHO, this was my favorite of all the tube preamps that I've owned. This and the MC240 was magical. Stupidly, I traded it for an MC2500 in '96 or so. I wish I still had it! 2nd place is a distant 2nd.
McIntosh MX110 - I ran this combo for the longest, 5 years or so, mainly because the very first stereo I ever had was an MX110 and MC240. Very enjoyable. Probably the most popular combo sold by Mc dealers at that time. My beef with the MX110 has always surrounded it's severe lack of inputs. The Tape Head and Xtal phono inputs are nearly useless today.
C29 - Honestly, the C29 was maybe a tad better than the MX110 in some areas (phono preamp and flexibility for example) but I didn't use this combo long enough to make a solid case for it over the MX110.
C20 - I bought a C20 / MC240 / MR65B from a gentleman when I was into flipping. The MC240 was the early version and the C20 had the brass faceplate. The C20 was like stepping back in time from what I was accustomed to sound wise.
I still do have an MC225, although it gets zero play time. I used it briefly with my C100 and again briefly with my C200 with excellent results. I'm not a fan of the Home Theater MX units, as they have some reliability issues. Twiiii and others rave about the MX119 and MX151 though so they can't be all bad. Personally, I'd be looking at STEREO preamps only, but that's just me.
Good luck, have fun, and don't be in a hurry. Let the right piece find you!