Responses from knotscott

How does one weigh the virtues an older amp vs. a younger one?
Better to listen to each for extended periods (weeks even), then decide which one you prefer to live with.   You miss a lot of the little stuff doing quick A/B listening.  
Preamps can color sound considerably. Surprising?
The only "authentic" sound is what the guys in the studio heard. Bingo....the true meaning of "Hi-fi" refers to the original sound as recorded.  Everything downstream of that is variation of some sort .... we just get to decide if it's a more p... 
Phono Stage Upgrade
It'll be difficult to do better than the phono stage in the Convergent SL-1 IMHO.  
Best 12AU7 Type Tubes for a Preamp
RCA clear tops and new issue Mullards sound good and have worked well for me.      
Who Remembers the DAK Catalog?
I do remember that, but it took your post to jog the memory.  
12AU7 tubes for a Rouge Audio Sphinx V3 Integrated
RCA 12AU7 clear top have a fairly large happy following. I’m using them in my Dyna/VTA 70 amps, and have been quite pleased. Those might be worth a try. Also available under the Conn brand.  
Should I bi-amp or bridge my amps to feed my speakers
I would not bridge your amps.  It forces two different output transformers to work in series.  Bi-amp should help with separation and soundstage. Add an active subwoofer to handle only the bottom octave. Set the low pass crossover... 
Class D for Repatriated Merlin TSM MMI
Hearing the name Merlin makes my heart smile.  A blast from the past for sure.  Bobby P. would be so pleased to know that his speakers are still being enjoyed, and that people have such high regard.  Good luck with your TSMs.  😎  
Finally made the transition to a tube preamp!
It’s always subjective and there are many variables, but given the quality of your system chain, I’d think that the retipped Denon with the ML profile and boron cantilever would be worth having in your system. I’m not certain, but I’d guess the PA... 
Single vs Bi-wire Speaker Cable
As always, there are several variables involved. Some systems simply reveal it better than others, and some people are more patient when can be subtle and hard to notice, but once you do hear it, you won’t "unhear it". It's more lik... 
Finally made the transition to a tube preamp!
Where can I learn more about that arcane skill you call "roll different tubes to tailor to the sound to your liking"? Tube rolling is optional of course, but it’s unique to tube gear and adds an element of fun and interest. Many/most (at least ... 
Where do you want your system to go when you go?
My oldest son has already inquired about it. He said he has vivid memories as a little guy of me building speakers, listening to music, and tinkering with the system. He’s shown ample interest, has started to put together a decent system of his ow... 
Finally made the transition to a tube preamp!
Welcome to the faithful believer’s club. Transcendence shall be yours. 🤣 I've been listening to tubes for over 30 years, and have no plans to backtrack.  I listen to tubes for their clarity, spatial separation, micro dynamics, and uncanny glimpse... 
The thing about objectivists is...
That's a literary marvel right there!  (and I agree with you)  
Which component to upgrade 1st ?
You've got the foundation for a very nice system.  I'd be  inclined to experiment with some tube rolling so you can tailor the sound to your liking...both pre and amp tubes. If you do, be sure to give ample burn-in time before you draw any conclus...