Has anyone upgraded from Merlin TSMs?

Celtic

Can you expand on your last paragraph?

Also, maybe I should upgrade my power. Presently using original version of the Billie amp. Thoughts?

Never owned the TSM, but I ran VSM-MM for well over a decade, still have them only not in main system. I modified both internal crossovers and BAM, I brought the Duelund caps to Bobby's attention. So modifications are possible if your TSM running the Hovland caps, you can also upgrade the resistors to Duelunds as well. Beyond that the only lacking in both Merlins is lower bass which a pair of Rel subs would provide the coherence Merlin so well known for.

Merlin coherence is amongst best out there, imaging as well, the Duelund mods will bring greater resolution, transparency and more natural timbre. Add the bass with the Rels and you'd be hard pressed to beat that combo.

Billie amp with the Duelund mods could be very nice, but I always found all tubes pre and amps to work best. For me it was EL34 push pulls and 845SET that provided best sound quality.

As just mentioned, all tubes seem to work best with Merlin’s.  I haven’t heard the Billie but on paper appears to be something that could scale up with the Merlins.  Have you tried rolling the tubes?  Some nice NOS 12au7’s might make a BIG difference. For NOS  I’d look at some Mazda, RCA, Amperex or Mullard.  All will sound different but depending on what tubes you have now can make a big improvement. I’m breaking in some Psvane new release 12au7’s now in my pre and combined with a class D amp sound very, very nice as well, particularity for the $$$ invested.

Sure.  Improvements in source, preamp, power, power conditioning, equipment stand and cabling all brought improvements to how the Merlins performed.  Some transducers peak quickly, others reflect any true improvements in front of them.  

Even quality massed stands brought improvement.  Best success.