HIGH PASS FILTERS: getting rid of the mud and rumble
If you lurk in on-line bass forums you'll likely see posts discussing the need for High Pass Filters. This useful feature deserves a specific explanation of exactly what they do and why you might benefit by having and using one. Simply speaking, they are filters that let the highs pass (and that's where the name comes from) above a certain frequency but gradually block lows below that point. Why on earth would we bass players want to block lows?? When those frequencies are below our lowest bass note. Think of a high pass filter as a sort of adjustable sub-bass tone cut control.
Upright basses can particularly be a problem to amplify (like you didn't know THAT), as pickups can often generate all sorts of rumble and mud along with the desirable bass tone. This occurs not just as we play, but also as the huge hollow body captures all the sounds in the room. (Try it sometime, leave your amp on and put your ear up to the speaker; you'll usually be able to hear sounds from the room that you are amplifying!) There are a number of reasons why this can be a problem.
Those low lows muddy up our notes, hurting our clarity, because the speaker is trying to reproduce those very low frequencies. That interferes with the notes we want our audience to hear. Another big negative is that unneeded lower frequencies eat up watts, power that we need to be heard clearly, especially when we're nearing the edge of what our amp and speaker can deliver.
Using the filter: It's always best to adjust filters and tone controls by using your ears rather than numbers (frequencies like the 41.2Hz of the open low E string). Your control may differ, but most have a knob that chooses the frequency below which the filter begins to roll off the sound. This is important to know-- these and similar filters do not have a sharp cut-off (like a notch filter), the control won't cut the frequencies off below that point like a knife — it's a gradual reduction. You'll often see specifications of -6db or -12db per octave.
Ok, those are the numbers, but as I said, it's best to use your ears, and make teeny-tiny adjustments to the frequency control until you arrive at a good compromise between thickness and clarity. The High Pass Filter can be a real sanity-saver when you're stuck in the walled back corner on The Stage From Hell — you know, the hollow plywood kind that vibrates, resonates, and makes everything on stage sound muddy. The control can benefit all bass players, not just URB. (I play bass guitar gigs on one of those stages from hell regularly.)
Don't have a high pass filter on your current amp but still want to overcome the mud? There are a number of external preamps and devices that could help, either with specific high pass filters, frequency adjustable (parametric or semi-parametric) EQ sections, or the tone controls you already have. But it does require a pretty good set of controls to really do the job. For instance, I use the lowest control of the semi-parametric EQ (tone control) section on my Euphonic Audio iAMP800 at the lowest setting to help clear the mud. Since it has such a wide range of frequency control, you can adjust it to the lowest frequency ranges -- and cut it pretty severely -- to approximate what a HPF does. It's not perfect, but it can work in a pinch. Turning down the bass, though? Yes, I know we bass players instinctively do the opposite, but fight the urge and don't be afraid to roll the bass control on your amp to the left a little bit. Very often that satisfying fat tone on stage turns to mud out in the audience. Reducing it can actually "tighten up" the bottom and make the bass tones more satisfying, ironically.
These filters can be called the different things on various products. For instance, the Fishman Pro-EQ Platinum Bass DI calls it a Depth Control. Both Acoustic Image (on all Series IV amps) and Euphonic Audio (on Micro and Doubler) do label it as a High Pass Filter.