Best tube integrated with Merlin TSM-MMI?

20 watts, which seems to be the Due Venti's rating, is likely not enough for the TSM-Mmi, in my experience. The Stingray offers the option of 18-20w in triode or 36-40 watts in ultralinear. As much as I like the sound of triode power (I also run a 2A3 amp in another system), I found it to be inadequate in my admittedly large room. The midrange is wonderful but the top and bottom ends get a bit ragged. Ultraliner, though, sounds extremely good through the whole sonic band. Of course, YMMV.

21 watts for the Primaluna Dialogue Two (38 in ultralinear) -- not much different than the Mastersound Due Venti.

VTL IT85?

I think 38 watts versus 20 watts is quite a bit of difference, same as 100 watts
versus 200 watts - 3db louder - and at the lower limits of what the Merlins
need, just might make all the difference in the world. I would ask Bobby, but I
think 20 watts would leave you feeling underpowered unless you have a smallish
room and listen at moderate levels - it could work, but not ideal and it might
always have you thinking, what if.....Unless the Mastersound has really high
dynamic headroom (6db or so) in which case it might be ok. That is a very
interesting specification you rarely see these days, one of the reason the old NAD
3020s worked with so many speakers, though only 20 watts, high dynamic
headroom.

38 watts is ultralinear spec for the Primaluna Dialogue Two. Mastersound is 20 watts Class A. Don't know about the headroom.