Mt' R: I have had the 3 hole version for like 15 years now (1994 production code). I have found it to be a solid performer.
The present day 811a's are not the tube of old. The old RCA/Sylvania's and GE's were much better tubes, however the present day
Taylor Premiums/Machlett/Penta's are not bad (all made in same factory) Quality control is much better since Penta took over watching the production. I would stay away from the "Generic" no-name versions, the failure rate is about 25%.
The Svetlana's were the best, but no longer in production, The Russian G-811's are about the same. Those brands were made for Military and much better QC.
The present day 811A's don't have the same Plate/Grid/Filament material or strength as the older versions. However treated properly will last a good long time. I try to keep drive power in the 50w PEP range and the amp will yield a good solid 500w out with Grid current well with-in specs. The 4 whole version maybe 10 watts more will yield a solid 600+ watts, yes Ameritron say's 800w but is that little difference worth pounding the tubes...NO
Now as to 572B swap, this tube is much stouter construction wise than the 811A. The so-called extra output comes from the fact, most drive them harder due to better construction. The output difference, no one will notice on the receive end.
They are a better tube however in my HO, worth twice the price: that will be your budget debate.
I have had a set of Svetlana 572B's in mine for at least 6-7 years, have literally beat them to a pulp. Several Field Day operations, couple contests etc... They were around 600+ watts when installed, after all this time still 450-500w+ all bands.
I have a NIB set of Svetlana and another set of OLD/USA made NATIONAL tubes (4 each) 811A's here in-stock when ever the need arises to swap them out.
I will most likely keep my eyes peeled for a set of Machlett's or Taylor premiums 572B's at Hamfest's or the Swap sites that are priced right and purchase those for additional stock. I have no plans to get rid of the AL-811 amp anytime in the near future.
I like NOMADE suggest operation at 240v, the power supply does exhibit better voltage regulation at that voltage than 120 volts.
I have however run the Field Day operations at 120v and they operate fine, they just exhibit greater voltage swings when loaded down.
I do have this one observation, Ameritron uses the SAME power supply in the 3 tube and 4 version, this is not to say the 4 tubes version lacks in power supply. Just interesting they produce both amps with same supply, and just leave one tube out.
There you have my 2 cents worth of info, both are good solid amps when run conservatively and will last many years service.
All the Best
Gary
PS: If you plan ANY AM Operation the 572B is your only real choice (at least the Taylor 811H/D version) and output in the 50% of rated PEP SSB operation, otherwise you will KILL them quickly!