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

The late designer of Merlin almost exclusively used Joule-Electra OTL amps at trade shows. You might want to contact Rich Brkich of Signature Sound, as he knows Merlin speakers as he worked closely with the designer.

Sure- the Jouls is known to be a favorite match by the designer, however, im trying to counter the speaker's philosophy here, so it is my thought that the Jouls is actually going to emphasize the leanness of the speaker, not counter it. Or Bobby P wouldn't have used it.

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. 

Try a McIntosh tube amp.  Very warm sounding. 

The most popular speaker for many years with our M-60 amplifier was the VSM. Very nice match and we showed with Merlin at several CE Shows.

But I would not call the amplifier fat sounding (which usually describes an excess of 2nd harmonic distortion which our amps tend to lack). It just sounds natural.