What is wrong with a sub?

first 2 subs are a lot better. I have 2 and have experimented running just one. No comparison  The Achilles Heel  is when come with a built in plate amp with volume control and cross over point. Most of these are crude and cheap

Emerald Physics make a interfacing product called The BOM (bass management) which works pretty well for ~ $300, but to get the best Marchand offers excellent outboard active XOs, but they are costly

David, I am hesitant to use different makes and models of subs when putting multiple in the same room. I’m not saying it can’t be done or that it can’t sound great but I believe you stand a better chance at integrating them when they’re all the same model. If you already have multiple subs and want to try it out, let me know what your results are. 

As for DSP, it stands for “digital signal processing.” Some subs have it built in. With my system I added a MiniDSP inline with the signal going to the sub amp. It lets you tweak the signal in virtually unlimited ways based on what programs you upload to the DSP. I use it to cut and boost problem frequencies which are caused by my room modes. You can also adjust the phase, time alignment, delay, add filters, high pass, low pass, etc. 

1+ wolf_garcia. mkgus as long as you keep it symmetrical you will be ok.
If you use two subs up front they should be identical. If you were to add two more subs they could be from a different manufacturer. The DSP is great but you have to know what you are doing and to do this you need to be able to measure the situation with a calibrated mic. You can not change the nodes in the room. You can only correct the frequency response at your listening position. If the listening position is in a nul point
you want to move it back and forth a little in the room to where the bass is if anything too loud. In a nul point it will cost you a lot of power to correct. This is easy to do. Get a test record or download with frequency tones. Play a 40-60 Hz tone and walk back in forth in the room right between and 90 degrees from the speaker axis. You will hear the bass come and go as you walk in just a few feet! Mark the spot you like and move the listening position to it. Now correcting the response will save you power and you won't need a 2000 watt amp.

I within last year discussed subwoofers for music with a Wilson and REL dealer, who said that even the Wilson ALEXX, a phenomeonal $110k loudspeaker rated to 20Hz, benefits from well-done subwoofers.  The benefit is subtle, but it's there, according to his crew as determined in A/B testing in their listening room.

There are many good subs on the market. Just get good controls for the subs and consider how many you really need and placement. I have speakers that go below 30 but I tried one good sub and then, over the years I moved slowly to four. There are reasons for this but you will learn them on the journey. IMO there is a better sound with four subs than most other setups. And I am not talking wall shaking; I am talking immersion and realistic sound.