Merlin VSM and SETs?

Pabul, I can't compare the Joule amps directly to the Carissa as I didn't use the Joules in my system. I can say I prefer the Carissa in my setup to the Joules in the systems I've heard, I suspect my fully treated room had much to do with this. I much preferred the dimensionality, holographic presentation over the mostly untreated rooms of the Joule systems.
I can't imagine you would have any bass issues with the Joule LA 150, I use the Joule LA100MK.III and bass is just fine for me.
You will only know if an SET is better for you by trying it in your system, I can guarantee it will sound different than any push/pull or OTL amp.

Pubul, dual output in a "quasi-parallel" circuit according to the manual.

Pubul:
I also use Wyetech Sapphire monoblocks: 18wpc parallel 300B SET with no negative feedback. They drive 88dB B&W Signature 805 monitors effortlessly in my 13'x19'x9' room with the same type of music you listen to. Stunning combo leads me to believe the Merlins would mate well.

I'm googling Wytech as soon as I finish typing this. The 300B has me a bit concerned regarding bandwidth, but I know the circuit design is important too.

I use similar sensitivity Acoustic Zen Adagio, 89 db, but equally important, a pretty flat 6ohm impedence. A benign impedence is probably more important than sensitivity with a SET. They sound just fine with a 22 watt Viva Solista 845 SET, real grip, good base and dynamics. I would say for 89db, at least 20watts, unless if you stck to Singer Songwriters, small Jazz combos. Me, I like a bit of Wagner and that certainly needs 22watts