Krell FPB-600 or FPB-700cx - RCA vs. XLR Input

Hi,

I am looking to buy a used FPB-600 or FPB-700cx amp. I am wondering if I should stick with the FPB-600 because I will be using RCA inputs. I know there are adapters to convert to XLR, but is that a good idea? Isn't there an impedance mismatch or signal level issue? If that is the case, I would not be able to get the FPB-700cx to unleash it's full potential? One thing I will not do is buy a preamp with balanced or CAST outputs. Thanks for any knowledge on this!