Low Frequency Distortion in Chanel 1?

When I got my new PRS custom 24, I noticed a low frequency distortion on channel 1 in all 3 modes even though it's EQ'd for crystal clean. It's evident on the low E & A strings and it diminishes as you pass the 12th fret. I changed all my power tubes which brought a lot of focus to my sound, as it always does, but the LFD was still there along with a lot of other sizzling and buzzing. So I finally changed all of my preamp tubes after many years of great performance and cleaned up the sizzling and buzzing, but the LFD remains. What could be causing this low frequency distortion in channel 1?

Sounds like the power tubes going. Power tubes can get farty when worn.

Thx. But I just replaced them all with Mesa tubes.

I have the low end set really LOW ( old school boogie settings ) on my MKV on the clean channel ( 2 or 3 ) keeps the bottom end nice and tight and lately I have been using the preset EQ rolled in some.