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

This thread does a disservice to the uninitiated by recommending SET and tube amps providing less than 10 watts of power without knowing what kind of speakers are being powered.  If you're new to audio please do not just go out and buy an amp based on a recommendation here without considering system synergy and speaker sensitivity.  

For example, I once purchased a Pass Labs XA25 that someone else recommended in this thread and with my 95db sensitivity speakers the XA25 sounded veiled with rolled off highs and loose uncontrolled wooly bass .  Sure it was warm and offered a tonally denser midrange but I hated everything else about it compared to the other amps in my stable.  I couldn't sell that thing fast enough.  

Pass XA30.5 to my ears is the best I’ve owned. Detailed and layered with great tonality. Refuses to be clinical or piercing which I appreciate. 

An integrated wouldn’t be my “end-game” amp.  Get a non-functional McCormack DNA 0.5 for around 500 bucks and send it to SMcAudio and have them rebuild it for you.  Absolute SOTA amp on the relative cheap, and then just buy the preamp flavor of your choice to go with it.  I use an LTA MZ2 tube pre (with upgraded LPS), but that’s me.

The XA-25 is an astonishing sounding amp ("veiled and rolled off highs with wooly bass" are absolutely NOT part of the XA-25 sound in my experience) and I’ve enjoyed mine tremendously for a few years now. It’s a keeper, as is my Had Firebottle HO (high output...ha...) SEP...that thing cost me about 1100 bucks and it was only a few months old (maybe 5 years ago). I swap between the two amps as my mood dictates and use very efficient horn speakers, so there’s that. Had and Pass make integrated amps from these two designs which are also well regarded.

Krell-Evolution 402e. Cost is very dependent due to condition of this near SOTA power amp.

Norma Revo IPA-140. Might be able to find a late model version for $4k which meets the OP's criteria.