???What's The LEAST Expensive "End Game"Amplifier You've Ever Heard???

Depending on your speakers a true end game amplifier that you wouldn't want or need to ever upgrade is the SE84UFO25 by Decware.  Two of the most musical watts out there.  The Decware owner actually made the first amp for himself as a state of the art  SET amplifier.  It turned out so good that he offered at over three times the cost of the SE84UFO and now the Decware backlog is an over three year wait time. 

It's a niche product for people with speakers in the 100DB sensitivity range but is truly a piece of equipment that you can inexpensively tube roll and tune for the specific sound characteristic  you desire.

I agree with you @danager! Those willing to DIY can bypass the waiting list and build their own with a printed circuit board and instructions from Steve Deckert for only $45.

https://www.decware.com/newsite/SE84CDIY.htm

the whole project cost me a bit under $600 a few years ago.

While I have bought a used set of PS Audio, BHK300’s and their Signature preamp, I still have a Hegel, H390 integrated and if bought used is one heck of a deal! 

An easy recommendation is the Atma-Sphere Class D Monos.

At $5400 a steal. As good, or better, then A-S tube amps.

To do better I had to spend $43K on a Dartzeel integrated.ll

I wouldn't (at this point in my journey) choose and integrated amplifier for myself, I prefer a separate preamp and amplifiers; specifically a tube preamp with both tube and SS amplifiers.....at least in my main system.

The Hegel H590 (now H600) would be that choice if I had to choose. An acquaintance in my audio group owns one I get to hear on occasion, and it really is a wonderful piece of gear. I might even be able to live with the built in DAC, if not it's streamer

If I was putting together a reasonably priced system for a garage or office, I could absolutely live with a Heaven 11 Billie Amp MKII. At $2,000 it's an absolute steal, and wonderful integrated. It appears the Billie MKII is sold out, and there is an upcoming Billie MKIII coming out soon which will be modular. You can even choose your output engine, Hypex or ICEPower.....will be following that release later this year