That is "music to my ears", thank you for that great run down
Did you try the Hifiman HE-Adapter? It looks like Dyson Audio has some short jumpers I could use to wire up the Hifiman HE-Adapter (a speaker terminal to XLR box with protection built in).
I don't see a specific XLR to speaker terminal SKU, did you get a custom one or build it yourself?
How about pre-amplifier 12AU7 / drive 12AX7 tube choices in the Primaluna Prologue 4? Which did you find most enjoyable? Why the KT120 vs other choices you tried?
I use this but with banana speaker connectors instead of spades (you can specify bananas, spades, or bare termination at checkout): https://www.ebay.com/itm/Dyson-Audi...phone-Cable-Speaker-Tap-Adapter-/114729319765
I then just connect a balanced cable directly to the adaptor and headphone. I suppose the Hifiman connector would probably be better since it has a protection circuit built in. I'm just really careful with the volume level and make sure the volume is all the way down on the sources when plugging the headphones into the adaptor. It works great and I haven't had any issues.
I like the way the KT120s sound with it so that is the main reason I'm running those. I do have a stock of NOS Mullard EL34s FX2s, NOS Tesla EL34s, NOS Wing-C EL34s, NOS GE 6AC7 fatbottles, NOS EI EL34s, NOS GE 6550s, NOS Tung Sol 6550s, NOS Wing-C KT88s, current production Gold Lion KT88s, and Shuguan Treasure KT88s that I can run with the Primaluna. I tend to use those tubes on other tube amps I use in my main speaker system. I like the fact that the KT120s are current production so when they need to be replaced they won't cost me an arm and leg (compared to the NOS tubes) to replace yet they still have great sonic characteristics (particularly with bass slam).
As far as the input tubes I'm running RCA 5751 Triple Mica Blackplates in the 12AX7 positions and CBS-Hytron 5814 Blackplates in the 12AU7. I find this combination to be exceptional and run this in my other tube amps that call for 12AX7s/12AU7s all with great results.