Daniel,
To start with, you have a very nice system now. I too have relied on modifications to maximize the sound of my basic components.
Interestingly, my conrad johnson PV 12L was extensively modified by RHB Design which is Bob Backert's modification company. He and his son do their magic on gear of various brands, although Bob has specialized for years in working on cj gear.
When I saw Backert Labs' two preamps on your list of possibles I was moved to reply. Bob's thing is attention to every blessed detail in a design. Impeccable workmanship. I've had the pleasure to meet Bob several times and he has the perfect temperament for what he does. He is quite meticulous and real low key. Patience!
I haven't had the good fortune to hear any of his proprietary preamps but if you are looking for a rich, tubey sound, his gear probably won't give you that. He is all about accuracy to the incoming music signal. Don't get me wrong, that "rightness" that a tube preamp can bring to a system will be there but I would doubt it would be generally characterized as warm in the classic tube sense.
Having said that, I wouldn't trade my Backert modified cj pre for any other preamp.
One last thought. Bob has patents on preamp power supply designs. He puts his all into the power supply section of his designs.
I have a Modwright SACD player with the tube rectified PSU. That PSU is a true key to the sound of Dan's modded units as I am sure you already know.
I look forward to reading about your preamp journey. All the best!