Peavey Bass Amp TKO115s
Start with a clean setting - zero cut and zero gain on the Eq.
Play a song close to the amp, and then move away, playing the same notes. Do you still get the distortion?
Change lower the input gain and repeat the test.
Lower the output gain and repeat.
Are any outboard distortion pedals in the mix, such as overdrive?
Eliminate them. Run straight through from your axe to the amp.
Check for any distortion settings. Set everything including reverb to zero. When the distortion disappears, this is your base setting.
Change only ONE parameter at a time. When the distortion comes back, you have found your problem.+