@jsqt You are indeed fortunate to have a Backert preamp. I have had the good fortune of knowing Bob for a couple decades, long before he made moves toward offering his own designs. Bob was known then for modifying preamps and to a lesser degree power amps. Most of Bob's work back then was upgrading conrad johnson units. He liked working on cj's like my PV12L because their was a lot of unused real estate inside those units so he could readily add his own proprietary circuits to improve their sound. Bob has always embraced a "purist" philosophy with an emphasis on the power supply. (Greenforce) Bob's circuits opened up the sound of cj's as cj gear is known to have a warm, rich sound. Very listenable but would obscure fine details. Bob's goal was, within my budget, to reveal all of the details in the music resulting in a more spacious soundstage when one is present in the recording. A year ago I took my modified PV12L to Bob as the volume control and selector knobs were creating a scratchy sound as I used them. Bob cleaned the knob contacts and then after asking him if there was any other circuit improvements he could install, he informed me that he could install a new gain stage dynamic linearity circuit that would be a significant step up. Done.
I wish I could hear my cj as it was before all of Bob's work and how it is now. I am quite certain the openness and musical accuracy of the modified unit would be obvious and desirable over stock.
Back in 2019 in this thread, I mentioned my embrace of Herbie's Tenderfeet. They are a nice improvement but once experiencing the musical clarity of the Isoacoustic Oreas, I now have every piece of kit sitting on them and the Tenderfeet are in the closet.
BTW...The Mazda CIFTE tubes really did sound good with Bob's circuits. The pair I had just didn't last as long as I would have liked and have been using matched pairs of Gold Lions yielding fine, balanced sound.
PS From everything I've heard about the Luxman M900u, I can imagine that it would mate well with the Backert pre. I have a Luxman phono stage. Thank God there is a vinyl revival in progress!