I'd also recommend XA-.5 over XA or X-.5. You may not have to get XA-100.5 to get adequate power for your speakers. For example my XA-30.5 clips at around 200W at 4 Ohm load. It's driving a pair of JM Labs Electra. You may or may not need more power for your CLS, but keep in mind that Pass amps are rated very conservatively.
Pass Labs amp choice for CLS - What would you do?
After much reading and soul searching I have decided to upgrade the amp on my CLS speakers. I can buy a used X350.5 (current generation) or a pair of X600 mono blocks (previous generation - can't afford the X600.5) for a similar price. Is the .5 model change significant? There is no way that I can audition/compare them on my system, so I will have to take a leap of faith. I know I can't go wrong, but what would you do? And what pre-amps should I be looking at?