I bought a demo C2600 and the dealer stated the primary difference is the DAC - it's modular and the C2700 came with the the newer version.
I haven't listened to them back to back; however, I have not experienced any discernable difference between the performance of source items (DAC and turntable) between auditioning sessions at the store and my house when recreating my system with the C2700 versus the C2600 bypassing the DAC.