I have a CC257 that has an OP24 Plus (4 sliders/3 buttons), a 1751 Balladeer with a OP24+ (1 knob/3 sliders), and my 1868T with the OP-Pro (4 sliders/tuner). I play the 1868T the most, but prefer the OP24 Plus on the CC257.
My gut feeling is you are correct that the OP20 is the better preamp, but before buying anything, take a look over at The Ovation Fan Club Forum
. They have a wealth of 'O' knowledge and plenty of great used O's in their classifieds. These folks, know their stuff and won't steer you wrong.