You don't say what guitar or amp or effect you are exactly using. I found for the few amps and EQs I checked that one should avoid any input or output level to exceed -12dB to prevent unwanted compression, soft clipping or even distortion. I use USB output and a DAW to check levels for input and output between blocks.

If you use a JTV with Variax digital input and Input 2 = Same the guitar input level will probably already slightly exceed -12dB depending on model selection, knob settings, playing style, and guitar setup. Use an unused input for Input 2 to lower your input level by 6dB, e.g., i.e., set it to guitar if you are using a Variax or Variax if you are using a guitar.

It seems the Mixer can produce output levels above the -12dB level w/o distortion, but watch out for effects after the mixer (they may not like a hotter signal tha -12dB). My experience is that if I send a -12dB signal to my DT it's way load. So suggested the -12dB limit should not get in your way creating a load tone.

You may also run into PowerAmp/Speaker distortion if you use a guitar setup for Bass (especially as you seem to get the worst effect for the low string).

Good luck,

Martin