If you have the LPF (low pass filter) and HPF (high pass filter) turned off, then the Frequency setting does nothing. The frequency setting adjusts the spectrum position of those filters.

If your subs are not as loud as you want them to be, there are a few possible things that could factor into this, and you should check them:

- Quiet signal coming from the source. For instance, the fader isn't set right, or the bass is turned down.

- The amp and speakers might simply not be as loud as you think they are.

- Enclosure problems, such as incorrect port size on a ported box, or incorrect volume on a sealed box. Or a leaky box, or a complete lack of an enclosure. They're in an enclosure, right?

- Wiring problems, such as wiring the speakers in parallel when they should be in series, or vice-versa.

- Wiring the speakers out of phase when they should be in phase, or vice-versa.

I think the latter is the most likely culprit, since you said that removing the filters made it the loudest. Having speakers in the wrong phase most commonly results in lowered bass response.