But it takes a lot of work.
Switches are noisy,
Power supply needs a lot of help
Circuit boards used cheap substrates and tend to lift, decay and sound bad
A nice, well restored PAS3 can sound very nice indeed, but a ratty old ebay one is likely to require lots of work. Of course if you think this stuff is fun (as I do) then it's a nice challenge.
My $.02
Hope this helps!
Doug