Long cable from MC amp to preamp.
could the signal be deteriorated in some form with 20ft of cable?
Is the mc amp a phono stage, which provides riaa equalization and a line level output? Or is it just a step-up, which outputs a moving-magnet phono type of level?
I would not want to run a phono-level signal through 20 feet of cable either before or after the mc amp. If the mc amp outputs a line-level signal, you may get away with it, provided that:
-- the output impedance of the mc amp is low.
-- the cable capacitance is low.
-- the shielding in the cable is high quality.
The only conclusive way to tell is to try it, of course.
What's your current phono stage? Unless this would be a big step down, you might consider getting a PS Audio GCPH phono preamp. with its variable gain, it should have no trouble amplifying an LOMC signal and driving a long interconnect, especially if it's low in capacitance (Kimber, Harmonic Technology).