Why do you guys pursue a flat frequency response when you buy a subwoofer?

Only able to speak for myself. I have a pair of MC275 in bridged mode, a pair Sonus Faber Olympica Nova V speakers, a pair of SVS PC-4000 subs, and to deal with sonic inequities in the room, a McIntosh MEN220.

The MEN220 requires some set-up where you enter information like the distance between your listening position and the speakers. Then the distance between the subs and you. There are outputs to the speakers and subs, each separate. you set the crossover in the MEN220 & I have no crossover set to the speakers, I do have the subs set to engage at the lowest frequency of the Sonus Fabers.

There are several other parameters to set in the unit. Then you use the provided mic to first measure exactly where your head is going to be when you listen & it runs through an information gathering sequence. Then you place the mic in various places around the room & let it collect that data as it generates complex sounds. It tells you when it’s done. You have it analyze the data and it adjusts the equalization for the room. The remote offers many options depending on the future listening audience including bypass. Once you use the MEN220, you never use the bypass option, the 220 is a magnificent device.

With that, when I’m listening to anything containing 35 Hz down, the subs handle that and there is an objective difference depending of what is being played. Listening to a movie, you feel the explosions, the subaudible crashes so often in the soundtrack thanks to the subs. Part of this is because the 1200 watt subs are vertical and the sound wave hits the wooden floors & the subaudible vibrations are obvious in my chair.

The subs, often play little part in most of the music I listen to, the speakers do a fine job of that, but, the subs do play a part and I’m glad they’re here..

@ka1j   What DAC are you using (DAC output going into the MEN220)?

Have you done a sonic comparison from your DAC output to the MC275 on your 2 main speakers (without subs) to the inclusion of the MEN220 in the chain and listening on the mains only? (without subs)

I use the DAC in the C2600.
Yes, the MEN220 is a new inclusion as is the 2nd MC275. I like to experiment and have tried the different connections, also settling on bi-wiring to the speakers.

The SF are absolutely stellar speakers and with no subwoofers, are beyond impressive. Running directly from the C2600 to 275 to the speakers insults the speakers in this listening room, is not a good choice, it is the room’s sonic problems, not the speakers, but is what it is. I can’t change rooms, I have to deal with what is here.

I wanted to see what the SF sounded like minus the subs and it is like being inside a piano but with an excess "resonance" for lack of a better word because of the room acoustics. Running the speakers alone through the MEN220 cleared up all of that. Adding the subs again, only adds to the lowest of frequencies, many of them subaudible & to the sound for dynamic effect. For example, listen to non synthesized music with just the subs triggering at 35 Hz and down to the 9 Hz these reproduce, you make out no words, can’t follow the music & really, I couldn’t tell what was playing. However, in Attack of the Clones, when the spacecraft is landing, with just the subs only, the house rumbles and the explosion when the ship explodes is like an earthquake, low sound and you feel it completely. Now add the speakers and play it back and you get the full experience. With just the speakers, you do not get the 9 HZ impulse or feel that vibration.

The subs have a purpose, they do not detract from the SF speakers in any way I can tell but they do provide an ultra low frequency dynamic. The MEN220 rounds it all to a beautiful balance that if I had a perfectly set-up listening room, I wouldn’t need it.
Your question asked about subs & I explained why I love mine, but also what helps that to be. I believe the MEN220 ties the ribbon onto balance between speakers and subs. Here’s a good review that explains the MEN220 better than I can.

https://www.tonepublications.com/review/mcintosh-men-220/

A subwoofer?   I am not on the subwoofer bandwagon yet.  I built my own speakers and I am not an audio engineer by any stretch of the imagination.  I am very picky about my sound but never had the money to buy the speaker sound I like.  My 8" towers when compared to klipsch towers where much more smooth, not as harsh in the highs  and gave deep lows.  Low enough I not only feel the punch of a low kick drum but dynamics that get ones attention and make me smile.  A properly designed set of speakers do not need a subwoofer.  If you have small speakers that cannot get those lows, or not the room for large speakers then I can see having one.  Subs in my opinion are great for home theater rumble, but I prefer a more balance less boomy approach.  I am sure many of the more expensive subs have smoother response on the low end, but I have no need at this time.  I personally am happy with out one.

I don't look for such a flat response spec, I look for indicators of good tone, fast, tight response. With Roon providing parametric eq options and many subs now offering similar via mobile apps its super easy to tune the sub to either your tastes or your room- both by eliminating boom, peaks, dips etc.