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

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. 

Interesting question because:  When we insist on a flat frequency response, then mess with it with the cheap crossovers most subs have, we are at least starting with a sub that does not add or subtract info from a poor design.  Personally, I use a pair of subs powered by a dual mono stereo amp, allowing manipulation of the volume without changing the frequancy response.  

A lot of people - just about all car audio guys, and a dismaying number of fairly serious audiophiles, are impressed with the slam of big bass and aren't really being honest if they say they want accuracy (although the ones on the audiophile end won't usually put up with too much boom that wasn't in the music).

Problem is that we are supposed to be trying to replicate a live performance in our listening room. Most people have zero idea what it would sound like if you put a guy playing an upright bass (much less a bass drum) in the corner of your listening room, nor how that would change if you moved him/her a couple of feet out or closer to the corner.

I am not sure that the average stereo fan IS looking for a flat response, although they might say they are.  Given that you don't get a flat response from the orchestra unless they are playing in a field, any structures within the sound field will affect what reaches your ears. All you can do is try and get a sub that is clean and doesn't start out by giving you artificial bumps.

A friend creates home theatres and sets in room response flat with pretty darned accurate instrumentation and tells me that he has NEVER - not once, had a client that didn't crank the gain on the sub channel.

Best we can do as home audio fans is to try and start with something that gives reasonably flat response and then try and modify that to eliminate variations introduced by the room. That takes good instrumentation, not guessing. I've been involved with that sort of set up, which can take several hours and a lot of speaker shifting about.

The speakers are a big factor - the main speakers in my main system are about the only ones I have heard that added no 'over-hang' in the low bass - a low note didn't fuzz and hang about, it simply stopped when the note stopped.   I had never noticed that over-hang until it wasn't there any more (on music that I was very familiar with). But I also think that it was more to do with driver design than having any sort of flat response.