From what I gathered surfing the WWW 5U4G is the only rectifier recommended for your amps in stock form.
I've only come across them with guitar amps of which a few local guys preferred the 5U4G (over the GB version).
This is of course apples to oranges as far as your AN's go.
Try and find the thread (threads) @ the Steve Hoffman Forums pertaining to your specific amps as there were gobs of posts/pics/info that I did not read through in detail (including power supply/slow start mods).
As they are kits I doubt that well thought out modifications would decrease their resale value down the road.
From the few pics I viewed they looked easy to work on (unlike my old Audion 300B amp that resembled the innards of my TV7 tube tester;-).
I would start by confirming that your units are actually built to the original AN specifications and then go from there.
Yes, it's possible that your old production tubes are duds as I've always found rectifier tubes difficult to test with my old TV7/Hickok 539 testers in that they always tested good until they didn't (same thing with 12ax7 tubes on the same testers).