HPF and LPF don't work... why?

[krash]

So my understanding is that the high pass and low pass filters are still "active" when the cab sims are off...
However, when I turn them on or off, I really don't hear any difference. In addition, how do I adjust the roll-off for hp and lp?
Isn't there an adjustment for where I want the roll off to occur?
Any high pass or low pass I've ever used, has an adjustment for frequency of where the hp or lp starts and the width and slope of the adjustment.
Also, what does the "reprise" setting do?

Thanks in advance...