I assume you’re trying some kind of “aux fed subs”. Question:
Where/what is your crossover? Is it in the amp rack, in an amp DSP or part of a powered speaker system?
You need to be aware that in such a setup the acoustic crossover point will vary with the signal level sent to the subs, rising (and possibly over-lapping your tops) as you increase the feed to the subs. This does not yield the best results and pushed to the extreme will cause some real trouble around the (attempted) crossover point.
If you have no crossover, all this is moot. You need one and attempting to use EQ and/or HPF doesn’t really work.
I would strongly advise you to use the Main L/R outs for your speakers, use a real crossover and forget the aux fed subs for now. If you need more LF in your setup, get more subs or turn your tops down to match the subbage you have.