Has anyone upgraded from Merlin TSMs?

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.

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.

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.

Celtic

Can you expand on your last paragraph?

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

I've the last version of TSM Black Magic with Master Zoebels on the terminals.  Great speaker and tough to match or even beat within the class.

I've also ATC SCM 20 passives.  Big leap in capital expenditure and decidedly better.  Why wouldn't they be?

The Merlins move well with improvements to what's placed in front of them.  Perhaps that's the best move.  Best success.

Merlin’s are still great speakers and still hard to beat for the price. I own the TSM Black Magic, the last version made before Bobby passed, and still use them at a beach house. In order to do better I went with QLN Prestige monitors for considerably more dollars. The Pulsars are a nice option too. But without upping the anti you probably would be chasing your tale.

Great speakers. I believe Bobby used Dynaudio drivers and think they’ve upgraded those drivers at least a couple times since then so time marches on. If I was looking to make a meaningful upgrade from those I’d go with a used pair of original Joseph Audio Pulsars that should be around $3500-$4k, and if they don’t offer a significant improvement you can sell them for little/no loss so low risk. Just one idea FWIW.  There’s actually a nice pair available now I believe in beautiful Sapele wood finish…

https://www.ebay.com/itm/186915696094