Depending on your listening habits, you could be well over 3000 hours on the pre-amp tubes since 2001. 10 hours per week for 6 years = 3120 hours of on time. 20 hours per week for 6 years is 6240 hours and 30 hours of on/listening time per week is 9360 hours over the course of 6 years.
Since the problem goes away when you place the pre-amp on mute, you know the issue is not in the amp. I think replacing the pre-amp tubes is the next logical step.
TIC