Thank you for the update cb3489. Those ACME 300B is not cheap but good to know it improve the X9 much.
I brought a pair of LINLAI 7300B. That pair also tightens the image compared to stock 300b. I didn't listen long enough to see how much those LINLAI tubes can improve over stock ones. Stock 300b is working well after I added the Denafrips Pontus II.
My X9 left channel lost sounds once a while. China HIFI wants me to ship the unit to Michigan for authorize repair shop to check. Instead of shipping, open the unit and solder the socket. The problem disappeared. While the bottom over opened, I checked wiring and noticed some wires and caps were very close to each other. I pushed its part and now very low noise/statics from the unit.