These changes without changing model number are not really that uncommon. I had an ARC LS15 preamp and noticed that a friends LS15 had a detachable power cord and different capacitors. I called ARC who confirmed this, and was able to have them upgrade mine. I have also owned two major audiophile brand speakers where the designer/owner confirmed changes made without changing model number and offered upgrades.
Pass Labs did indeed make a change, adding the "signal ground pin". I believe it's on all later X.5 & XA.5 amps. I think this is for grounding subs like the RELs. Ask your local Pass dealer, or just contact Mark at Reno HiFi. He'll answer any question on Pass.
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