How good are McIntosh MC7300 and C40?

The C40 is great. I've been using it with my MC352 and the C40 sounds much better than several other preamps that I also own. I'm now experimenting the C40 with a VTL amp and I can tell you they sound wonderful together too.

Yes the C 40 is one of the best. I also uses it with a 352. Nothing to say. It sounds better than much modern machines. The TD preamp section is better than more recents, so if you listen to vinyls, keep the C40.
Keep the 7300 too. You can find better... But at what price ? You will spend much (very much) money to win very little upgrade.
If you check for new cables, may be a power line curator, it could improve your system at rather low price.
That's what I'll do.

You are so lucky.Check out the Mac place in town as one day you may need to tuneup the 7300.The rest are practically resistant to all but bad luck[moisture,dropping,etc......]I have the C30-MC7270-MR77,keep on truckin.....

I have a C38 and MC7300 which I have owned since new in 1992 and I listen to my setup (Dali Euphonia MS5s, Shanling CDT 100 with upgrade by Mitch Singerman, Velodyne subwoofer, Music Fidelity Tri Vista Dac and Monarchy Audio DIP, Shunyata Hydra, Nordost cables and interconnects) every day. I do think I'll upgrade the preamp as others have mentioned but its a sweet sounding system. I'll be keeping the 7300.