Highest BUILD quality tube amps?

Coincident Speaker Technology, my 300b Frankenstein mono's are PTP wired and have been trouble free for many years.

I own both a Cary SLI-80 and an Ayon Audio Spirit III.
IMHO, Ayon is far far superior in build quality (for twice the price at new retail, so you’d expect that).
The Cary’s volume pot has become loose over time, and so has the power switch, and the original remote is starting to fail.
The Ayon has the additional benefit of tube auto-biasing and comprehensive startup/shutdown sequences that extend tube life. And it is a far more controlled and prcise sound than the Cary (which is still very pleasant when not making direct side by side comparison)
That said, the Ayon remote is a super solid shell of heavy aluminum, but the soldering of the board to the battery holder is weak and almost impossible to repair (I tried, failed and had to order a new remote).

Mastersound of Italy.
NAT Audio of Serbia

I have a pair of these:

https://www.vinylsavor.com/en/products/300b/

Tube Research Labs  ( TRL )
... God Bless Paul Weitzel ... 
Completely in their own exalted league.