Threshold, Levinson, Bryston, Krell, ?others? used amps

Just becareful buying used ,when it does died ,you have nothing but metal.Which could be an expensive piece of junk.

Krell ksa100. Pure class A. Doubles all the way down 2ohm. Beautiful sound and beautiful looking amp. A true classic. Might be had for 2k if lucky. 

In terms of value, I'd lean towards Bryston because unless you're purchasing something super old you're still going to be covered by the warranty and support.

In terms of the sound character you're looking for, there's a reason Bryston is used in so many professional audio setups.

Lastly, to answer your question about the benefits of the newer Cubed models over the previous generation amplifiers, the Cubed amps have even better detail retrieval, clarity of sound, tonal balance, and noise rejection. They just sound better all around, assuming the sound character described above is what you're looking for. I strongly prefer the Cubed versions over the previous generation.

Never heard a PA7 but knew the PA5... I’d sell the Nak as they still bring in decent cash and spend a little more and look at Pass X250. In my system my X250 disappears or actually my speakers do. I do use a tube preamp but I’d say my preamp is more on the neutral side than tubey or soft,  but that being said my system is smooth, but not slow, bass authority but the Pass doesn’t throw the bottom end in your face but it can slam when needed.

I’ve listened to many McCormacks over the years and the past and current Parasounds and they all are very good and I’d pick those over the Nak but no way would I trade any of these for a Pass... even an older X250. Everyone’s taste and system is different but don’t overlook a Pass. The X250 is probably close to your budget but they don’t show up too often but maybe Reno HiFi has one. Yeah it’s a little old, but built like a tank to last.

The Odyssey Stratos or Khartago, as previously mentioned, is a very good candidate. If purchased new, 20 year warranty. Tough to beat for the money. USA made.