Try the EQ in front with everything at mid-point, except for high frequencies. Cut frequencies that are above 6 kHz right off. The sliders between 2 kHz and 6 kHz cab adjusted to taste. The preamp is killing frequencies below 1.6 kHz in a couple of places. That makes the octave up to 3.2 kHz for the signal passing by, even though it's being suppressed.
If you drop the bass below 100 Hz by 3 - 5 dB, it will allow the frequencies from 150 - 300 Hz to be more dominant in the lows and the sound is more focused in the midrange, rather than being swallowed by the bottom. This will help with adjustments made to the top end by allowing it to be more open.
In the loop: the only EQ that absolutely needs to be done for the upper harmonics fizzing is, depending on mode, to cut around 2 kHz to 2.5 kHz by small amounts until it clears up, but retains the attack.
In the loop: depending on speaker, most frequencies above 3.5 - 4 kHz are rolling off and won't be reproduced properly. Frequencies past 6 kHz are not being reproduced at all. Experiment with cutting the upper treble bits by small amounts if you need to and you can cut more into the highest frequencies. The loop requires more precision than the front end.
