There is no difference between a pass amp built yesterday or 9 years ago. When a change is made the model changes. If the model has not changed then nothing else has either since the original design.
My comments are based on a conversation with Pass Labs two years ago. I had called to ask about the changes on the rear panel between earlier and later versions of the X250.5 The RCA and XLR connectors are located differently on later models.
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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