I really enjoy reading this forum so I like to give something back when I can. The following is my experience with integrating subs into my two channel system. This has involved several steps over the last several months. I'll edit this post to add to my story when I have time.
It was a forum post here at AA that led me to the following page by 'Soundoctor' about how to properly integrate a sub:
http://www.soundoctor.com/whitepapers/subs.htm
The new thing here for me was learning that there is a ~ 10 ms 'group delay' in subs that means they can (depending on placement) lag behind the mains by about a full cycle at 80 Hz. That also means that if you get them in phase at 80 Hz, they will be out of phase everywhere else (except multiples of 80 Hz).
My home theater system has a sub (JL f110v1) so I started by following the Soundoctor setup procedure to get the sub volume, phase and mains delay correct. This really works well by reversing the phase of the mains and looking for the null at the correct phase delay. The speakers are PSB Synchrony 1s, which are nearly full range. I then tried listening to many bass heavy music tracks using a 80 Hz crossover frequency or just the PSBs full range. Integration was so good I good not tell a difference on almost any music tracks (movie tracks are a different story). The sub did add some extreme low frequency extension to a track called 'Earth Drums'. The only place I can put my sub is right next to one of the main speakers, so sub was exciting similar room modes. At this point I was convinced that it could be at least possible to integrate sub(s) into my two channel system without doing harm.
My two channel system was: JRiver on Windows 10 Intel NUC -> Benchmark DAC2 via USB-> Benchmark AHB2 Amp-> Magico S1. About 10% of the time I felt I was missing something on the low end. If I was ever to use subs, I thought I would likely use JRiver somehow to deal with crossovers and room correction. So, next step is to see if JRiver DSP engine is transparent. I had the excellent Classe CP-800 when I was using the PSBs as a two channel system but the DSP was not quite transparent so I had reservations about JRiver DSP. I set a filter in JRiver that did almost nothing and then turned DSP engine on and off. I could hear no difference. I was always a two-channel, no tone control purist, but I really changed my mind on this when it can be done in the computer before any DAC. Therefore, I thought I might as well use it. I setup a Linkwitz filter to give the S1s a bit more bass extension. This was a positive step, but not quite good enough and the extra extension came with a bit more distortion in low frequencies. To anyone using JRiver - don't be afraid of the DSP engine! In a rational world, manufactures would distribute a convolution filter with their speakers.
To be continued ....