A short while ago I posted about this not being as necessary as some think and got some vigorous pushback. Here I expound a bit more. If I want to use a subwoofer in my stereo system can I use a simple preamp output to drive the sub or do I need "bass management"?
The answer is it depends. First off what is bass management? In an AVR (Audio Video Receiver) it is twofold. One is sending bass from all channels below their crossover points to the subwoofer. Second is routing all signals from the LFE (low frequency effects) channel to the sub only. In a stereo system bass management simply means a crossover. Some preamp and integrated amp subwoofer outputs have a low pass filter (LPF) for the sub and some have this and a high pass filter (HPF) for the main speakers). Some have a full range jack labelled subwoofer which is a full range signal summed to mono. Others have no sub output at all. All powered subwoofers have their own LPF for their driver and and a few have an HPF built in when one uses the speaker level inputs. Most high quality subs do not have speaker level inputs.
So can I run a full range signal to my sub and run my main speakers with no HPF? Sure I can. My subwoofer already has a built in perfectly good crossover. Also my main speakers are designed to be run full range from whatever amplifier they are connected to. I know of no speakers on the market that the manufacturer recommends they be high pass filtered to play properly.
The usual objection to this is that I will get higher distortion from trying to force the woofers in my main speakers to produce frequencies that they can't. This is mostly nonsense. They were designed to be driven full range. Anyone who has played a warped record with no infrasonic filter can tell you that any woofer is fully capable of moving in a pistonic fashion at 2hz, never mind 20hz. The problem with smallish drivers or those with limited excursion is that they don't make a very loud sound at very low frequencies. They move just fine, they just don't talk very loud. Woofer distortion happens at higher frequencies when the cone begins to break up (flex in an uncontrolled fashion).
Others will talk of intermodulation between the ultra low frequencies and the midbass or midrange (depending on whether you have a two drivers or more) This is already cooked into the design. If you like the way your speakers sound then this distortion source is not causing audible problems.
The final potential issue is blending the response of the sub and the mains in the crossover region. This is usually not a big deal. The natural rolloff of bass SPL due to woofer displacement (combination of area and excursion) is a pretty smooth slope. Adding another slope to this with a HPF is not necessarily going to improve things. I have a mid to low priced sub from SVS that has selectable LPF slopes of 6,12,18, and 24db/octave. I can match the natural rolloff of my Imagine T2's pretty well. If we listened in anechoic chambers, perhaps we would want exact matching of crossover slopes between the mains and subs. We do not and a couple of db hump or dip in the crossover region is going to be swamped by the untidy mess that bass response is in home listening rooms.
I write this as I am about to replace an amp containing DSP that I was using to HPF my main speakers and I am wondering what effects this will have. In real terms, nearly nothing. I will need to tweak the subwoofer setting a bit, that's all. Bottom line is if you are shopping for an integrated amplifier or preamp and plan to use a sub, it's nice to have a jack on the back to connect it to but you don't have to narrow your list to only those that have two way crossovers built in. If your subwoofer doesn't have left and right inputs (many do) you may want to use a summing network to create a mono signal from your preamp/amp. Ideally, you want the output you use to be separately buffered so you don't reduce stereo separation in the main outputs when using a passive summing network. Most, if not all, bass in recordings is already summed to mono at subwoofer frequencies so, although I haven't tried this, you could probably just connect one left or right channel to the sub and never notice the difference.