Doc, there is no upgrade possible for the Pass LabsX-600's to the new generation .5 series. I believe that the 600's were the best sounding of all the X-generation amps, but the new X-.5 series is a significant change/improvement compared with the first generation X-series. If you can get a pair of 600's for that amount of money, it would be a great addition to the sonics of your system. I have heard the Classe amps in a couple of different systems, but can't be helpful regarding if their is a significant sonic difference them and the PassX-600's because it was not an audition in my own system. In my system for years I used Transparent cabling, went through a long period of auditioning over five different cable companies wires and finally decided on Acoustic Zen IC's and speaker wire. It's all personnal taste and system synergy, but I believe the Acoustic Zen wires are significantly better then any of the Transparent stuff and very reasonable priced. I have a review of the new Acoustic Zen Absolute reference here on the Gon that you might find helpful. Hope this helps.
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