This is really a side comment and doesn't apply to main speakers vs subwoofer. I have found that frequency response down to 20 Hz is not necessarily that important, especially for music. There are definitely recordings which you can get that do go down to 20 Hz, but they are a rare item. If you really look at how most people respond to bass in music, the most important area is the 40-70 Hz area. That is where we feel and perceive most of the visceral impact of bass. The low 20 Hz rumble is nice, but if you have a hole or bass null at 50-60 hz, you will feel a significant lack of bass.
Will I benefit from a subwoofer with 20Hz speakers?
My source is a minidsp shd studio with Dirac going into Denafrips Gaia DDC to Denafrips T+ DAC to McIntosh 601 Monoblocks to Cabasse Pacific 3 speakers. The speaker's published frequency response is 41-20,000Hz. I presume this is achieved in an anechoic chamber. In my room however, it goes down to 20Hz, at least according to the Dirac measurements. In fact, I needed to flatten the curve and reduce by 5-20 DBs between 20-100Hz due to the room effect.
So, considering I already go down to 20Hz, is there anything else 1 or 2 subwoofers will do for my system? Would it create a more consistent low frequency field? I see many people adding up to 6 subs, so I wonder what I'm missing.
Thank you for your insight!