What type of phono preamp do you have? You may be able to make some adjustments that may help fix the problem.
Since the problem started when you added the sub, I don't see why the problem can't be with that also. How do you have it connected to your system? If you're using an LFE output on your preamp, do you also use the LFE input on the sub?
Since the problem started when you added the sub, I don't see why the problem can't be with that also. How do you have it connected to your system? If you're using an LFE output on your preamp, do you also use the LFE input on the sub?