I guess there are two separate issues here; Do you need to free up some finances or are you just unhappy about the (apparent) mismatch but like the sound you get at the moment?
Circumstances change and Naim have good residual value. It's therefore unlikely you'd loose much more by selling now as opposed to hang on to them and, perhaps, sell-up in a year or two.
Of course, if you're not happy with the system as is then there's no point in hanging on to them.
For what it's worth, I have only read reviews of the concept 20. They all seem to point to a fun, warm, big but not to tightly focused sound.
Based on this (and nothing else) I'd consider something clean, fast and transparent sounding. Exposure, Cyrus and Rega spring to mind. Each should enable you to put an amplifier/cd player combination in the region of a Unitilite or below, depending how far up you want to go.
regards