Whats the FATTEST sounding Tube amp to warm up Lean speakers?

I for one would not characterize the Quicksilver midmonos as "fat" by any definition of the word but it very well may flesh out the lean character of the mid/upper bass of these speakers which is where the issue lies IME.

Agree, that many stock vintage tube amplifiers could be characterized as "fat" particularly in the area of bass. vintage McIntosh and Conrad Johnson certainly come to mind. I also concur with Rodman999999 regarding those vintage British tubes to help fatten up the presentation. I know a LONG time Merlin owner that went from Joule 100 watt monos to a Roger Modesky Music Reference RM9 after the demise of Joule-Electra. Last time I communicated with him he was quite happy, something to consider as well. 

Hey Joneil, thanks for that update! As from others as well, the Music Reference are next on my on my radar if ever again the itch rears it’s head. Love my Quicksilvers for now and possibly ever as it seems you are with your RM9s, ain’t life grand!

Roger, David Berning, Ralph Karsten and Mike Sanders have been at this for decades, dilligently improving their designs with  each personally involved. This is what I place value on when considering any audio product, personal committment from the owner/designer.