Assuming your TT was working fine in your previous preamp and both channels are equally affected, do you have a digital source hooked up to the line stage? If so, turn it off and then run the phono section. If you get the same thing then it has to go for repair if changing the tubes didn't help. The only thing common to both phono channels is the HT supply to the output 6922 and it looks like CAT uses n-channel mosfet regulators to the plates.
CAT SL-1 Reference repair
I recently bought a CAT SL-1 Reference preamp. The line stage is great. But the phono stage doesn't work properly. I crank up the max volume but it doesn't give reasonable listening level and it became too noisy. The sound is dry and week. It doesn't balance well either. I replace all the tubes in phono stage but without luck. Anybody has a schematic of this preamp? I didn't find anything on internet. Any suggestion is appreciated. Thank you.