A high pass filter is something which reduces energy going towards the driver. It is done because, especially in a ported box, the driver and port get out of phase below tuning or something along those lines(someone else can probably explain better than I... I know very little about ported boxes). That in turn leads to some massive excursion issues(which btw, don't even give you much output, as the port is also canceling much of the driver output below its tuning). What you therefore want is minimal energy reaching the driver below the port tuning.
You can not make a passive version of a low bass filter... it's insane. It must be done ahead of the amplification stage, not after. A miniDSP can likely be programmed to do it(those things are amazing), and perhaps your computer software can be modded to do it. The computer may have an LFE out low pass filter, and it may not. It may have a high pass filter for your mains, and again, it may not. THe signal leaving your computer may be 1v. It's a lot easier to reduce a .8v signal to .5v than it is to reduce the amplified 28v signal into a 20v signal, just for example. It takes less powerful crossover components, and doesn't waste power. In the latter scenario, you're basically trying to stop like 100 watts from reaching the driver, in the former scenario, you're stopping like 3w from reaching the amplifier. This will also mean your amplifier will have more headroom and not be overworked.
My recommendation is to pick up an elemental design EQ2, a Behringer mic2200, or a Reckhorn B2. These are proper analog high pass filter devices.
Many plate amplifiers too have high pass filters built in. I know Creative Sound Solutions sells BASH amplifiers and for i think a $30 charge they will modify the HPF setting to your needs. I don't however know the slope of the crossover here. We recommend a steep slope in order to get the most performance.
For example, we may want the signal to be 24db down at 17hz in order to keep excursion under control. A 24db/octave filter centered at 25hz i think will give you this. If the high pass filter slope is 12db/octave, then you have to center it all the way at 50hz or something like that. I think you can imagine that you don't want the high pass filter operating in the usable range of your subwoofer. (IE above port tuning). Doing so will just give you less output with no real benefit. What we want is balance between the most output above port tuning and the least excursion below port tuning. Remember, if a port causes a driver to be 24db down an octave below tuning, and a high pass filter causes the signal to be down 24db below tuning, the actual sound will be luje 48db an octave below tuning. Does that make sense? Do we want the actual sound to be this down? Of course not. But it's a necessarry evil. The most effective way of avoiding these things is an infinite baffle subwoofer. This implies a TON of drivers operating in virtual free space (an "infinitely" large sealed box). We use many drivers in order to protect each individual driver from overexursion while getting a ton of output, and being a theoretical sealed box, the transfer function rolloff is shallow enough that we can get a ton of output. You can even EQ an infinite baffle to give you deep bass. The tradeoff of course is a need for tons of power and tons of money. Many other alignments do exist to give you proper deep bass, between ported, sealed, and transmission line boxes. They all still require the right drivers. For this particular driver, you need to follow annunaki's advice and go with the least of many evils.. a 4th order active high pass filter centered at 25hz.
I really doubt your computer has much natural bass management... it's likely just not programmed into the firmware. Remember, most PC setups consist of a bunch of signals being sent to a "control center", not directly to an amplifier. I highly recommend a miniature surround receiver... something like a marantz nr1501. Even then, the high pass filters it will apply will serve to protect your satellites, not your subwoofer, and the subwoofer will only be low pass filtered (IE frequencies above 100hz will be down in level etc). A proper high pass filter is still needed to protect your sub. Most commercial subwoofers have HPFs built in.
