My understanding, including from a personal email exchange with Pass himself, is that he designed the original MOSFET-based 5x00 series just as he'd done the original bipolar-based 5x5 series. And as others have said, he had no part in the modifications made for the mk ii versions of some of those models.
I have a GFA-5400, Japan-made version, that I picked up used about 5-6 years ago for $150. My local tech had completely gone through my 535 and my 565 preamp and replaced tons of caps. I paid him $40 to go through the 5400 and check it out, and he said he cleaned it but it was totally fine otherwise - caps all good, performing to spec as-is.
It is on my list to get it serviced one of these days, though, as those caps will need replacing eventually.
However, I am mighty tempted by the emergence of such highly performing Hypes-Based amps now coming down below the $1500 (or I guess $750 per channel) price point, and so I might just save up for one of those. I do love that 5400, though - I don't know if it's the MOSFETs or what, but I slightly prefer it to my 535 (yes I know they have different power capabilities as well).