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?
The X-600 amps are GREAT! I had a pair for 2 years and am a huge Pass Labs fan. BUT they put out TONS of heat. That is the only reason I sold mine. I went with EDGE NL 10.1. WOW! Way better IMHO. Way cooler, more detail, smoother, better imaging. Highs to die for. 1/2 the footprint.
Dchazen, take a look at my Gon reviews of the 350.5, XA-100's and XA-30.5's to get the details why I would select the 350.5 over the X-600's. I did not care for the X-series amps sonicly, but the X-.5 series are great sounding amps.

Srwooten, I'm glad that you love your Edge 10.1 in your system. I had an Edge NL-10 and enjoyed it very much, however when the new X-.5 series came out, it was much more musical then the NL-10 in my system. As stated above the X-series amps to me were somewhat cold and steely sounding to me, but when Nelson Pass came out with the X-.5 series and then the XA and XA.5 series he nailed it and I believe that he surpassed the Edge gear in its sonic performance.
My comments were only on the X-600's to help the OP.
Srwooten, I hope my comments regarding my experience with the Pass amps compared to the Edge was not taken by you as a put down or negative statement about you or your experience regarding the X-600's vs. 10.1, if so I'm sorry if it came across that way.

The Edge amps and the Pass Labs amps are some of the finest sounding SS power amps built today. It really comes down to personnal taste and system synergy.
Thanks for clarifying. No offense taken.

To the question between between X350.5 or X600. I suggest the you take a look on the specs of your speakers. If the X350.5 can handle it, much better. Don't over drive your speakers.

For preamp. You can go for Passlabs or look for a tube preamp with a simple circuit like EAR 834L chrome, Audio Research, VTL etc.
Thanks for the feedback. As I continue to talk to people and read it is becoming clear that the .5 model change is important. The CLS likes a lot of power and the impedance can dip down to 2 ohms, so the amp has to be stable. I understand that the original XA series had issues with handling low impedance speakers, but this was corrected in the .5 series. The pure class A power of the XA is supposed to be phenomenal sounding, but the class A watts are more expensive. On the other hand, I have read that you don't need as many class A watts to drive the same speaker. I have seen comments that the X350.5 and the XA100.5 are the "sweet spots" in the Pass Labs model line up in terms of power that can handle most speakers and bang for the buck. If the choice is between the X350.5 (MSRP $11K) and the XA100.5 (MSRP $16K), then what would you say?
Dchazen, if it's in your budget, in my opinion, you should go for the XA-100.5's. If you read my review on the XA-60.5's for the details regarding their sonic beauty, you will see that they are on a higher musical level then the X-.5 amps and yet now have the current/watts to drive virtually any speaker. I would also suggest you give a call to Mark at Reno Hi-Fi with any questions you might have and he's great to do business with. You could set up a 30 day home audition, if you don't keep the amps you pay for shipping back and a very small restocking fee.
I also recommend the XA100.5. In my review on Audiogon, I compare it to other Pass Class A amps that I have owned, and it is by far the best of the bunch and it can handle difficult loads. You will also have the advantage of mono blocks over the stereo X350.5. I have not heard the X350.5. Mark @ RENO might even have a good demo listed. I bought mine from him and it was in excellent condition and I saved some money. Two audio friends heard my XA100.5 driving my Egglestons and both called RENO and ordered pairs.
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.