Peculiar Hum Issue on Jolida JD9 Phono Preamp

I am experiencing hum in my phono preamp after upgrading several of the internal components. The rest of my analog equipment is Teres TT, Basis Vector 4 arm, Zyx Airy 3S SB cartridge, Linar Model 10 amp. I first noticed the hum after one set of upgrades whenever I connected the arm cable to the Jolida. I reduced it to manageable levels by changing the 12AX7 tubes in the preamp to 12AU7 variants (to reduce gain) and using a lower gain setting in the Jolida.
More recently, I upgraded caps in the input section and I purchased the Granite Audio phono burn-in CD. I connected the output of my Oppo 83SE player to the Jolida and now the hum is back. I used 12AX7 tubes for the purposes of burn in as I don't want to reduce the life of my 12AU7s.
If the RCA cables are disconnected from the CD player but still connected to the Jolida, there is no hum. The Oppo does not have a ground in the AC connector.
I know I can probably get the hum down to manageable levels again when I switch to 12AU7 tubes but I would like to try to eliminate this altogether. Any suggestions as to what I can try?