Pass Labs amp choice for CLS - What would you do?

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.

"If the choice is between the X350.5 (MSRP $11K) and the XA100.5 (MSRP $16K), then what would you say?"

XA100.5. But don't rule out the 60.5 or even the 30.5 especially if you don't play your music that loud and I'm guessing you don't with the CLS. Electrostatics are capable of such great microdynamics, an XA.5 should be a great match for them.

For a preamp, Pass's XP-10 or 20 would be a nice match and if you favor tube pres, Conrad Johnson is my favorite. As I'm typing this in my office right now, I'm listening to a CJ CT-5 driving an XA30.5. I've had no desire to change this combo in my office.

Thanks for all the suggestions. I spoke to Pass Labs and was told that the equivalent power amp for the X350.5 is the XA100.5, with maximum outputs of 28 amps each. I'm afraid that the smaller XA amps might have trouble driving the difficult CLS speakers.