In my Phono Pre I run GE 5751 Black Plates - Triple Mica’s instead of 12AX7’s and LOVE THEM… The 5751’s make for a great phono pre tube, might want to consider over a 12AX7.
I also run a set of RCA 5751 Triple Mica Black Plates in my DAC and they do a great job of giving it a more of an analog sound than other tubes I’ve tried.(Mullard Reissues, Sovtek LPS’s, JJ’s, Pisvane’s)
Something to consider…Could also be a mismatch of synergy with your system based upon your cartridge not liking your phono pre or vise versa… Although the Nagaoka MP-500 is an awesome cart !!! The 200 is my next cart !