Just spoke to Peter perkins at pass today and got an update on the upcoming X600.5. The engineering prototype device has now been making music for a while.
According to Peter, the team at Pass is now auditioning what may very well be the final tweak of the design. Everyone seems to be happy about the results, but the final go-ahead to close the engineering/design phase and transition to full scale production, will come only after a final audition/approval by the company CEO.
Peter suspects that full production may be under way by the end of May 2005.
I asked specifically about audible differences with X350.5. Peter answered that while there is a lot of similarity with X350.5, the X600.5 has even greater ease, liquidity, bass control, channel separation, and overall 'sparkle'. He suggests that the difference is not due to any design change, but to the complete separate chassis implementation and the dedicated power supplies in each monoblock.