Your experience of moving to two subs

I have 2 subs-10" Eminence.  We run these with a Yamaha power amp, commercial.  I have never ran out of power.  I have a Marchand  crossover to make sure the level is right for subs and mains.  The crossover is at 100 Hz.I don't have phase adjustment.  These subs are where they were for 20 years.  I don't know if it would sound better somewhere else.  Recently I removed the crossover and ran the speakers fill range. Not as clear, less bass and transients all were not as good.  Why?  The subs reduce load on main amp and reduce intermodulated distortion.  This causes the mids to be not as clear.  I  have 2 bass traps and 6 first reflection with 3 diffusion panels in rear.  I finally have the system so I can walk around the room and the sound is the same, except no imaging.  I could buy a couple more bass traps, don't cost much.

I've gone from 1 Rel to two Rythmiks and now adding 2 more Rythmiks.

Going from the Rel Storm III (vented) to two Rythmiks F12 (sealed, in heavy DIY box, aluminum cone) was definitely an improvement in every way. My audio setup is in the living room so ergonomics dictate plausible places for the sub/s - which aren't that many. So it's really no surprise that two worked better than one (and 2 servo-driven Rythmiks were cheaper than the Rel to boot).

Later I experimented with stereo vs mono subs (both subs playing the same signal) and didn't hear a "stereo" difference - I was crossing over at 60-70Hz. I used the Rythmik's xo for a while to relieve the main (tube) amp and speakers from reproducing low bass and that was definitely an improvement. I did get a measurement rig to correlate what I heard with what I measured and with what I read in papers/online/books.

Eventually I went with a fully active system and have the signal for the subs sent to a miniDSP 2x4HD and implemented MSO (Multi Sub Optimizer) and believe that was another improvement. With MSO you take measurements of each sub at each listening position you define and the software optimizes (amplitude, delay, PEQ) for the flattest response across the defined listening positions. So I used my two subs to simulate having 2, 3 or 4 subs at the different locations I could live with and ran the optimizations. That showed that 3 subs were better than 2, and 4 were better than 3. This is consistent with Earl Geddes' approach. He has said 3 subs would be enough, provided there is freedom to locate them, which I don't (all need to be on the floor, two visible by the main speakers and two under end tables). So I went ahead and purchased 2 additional F12 kits. I'm building the boxes now.

Still TBD if all this will be worth it, but it has been a fun learning journey 😀

I've just started playing with this in a PC system.  I'm buying small subs off ebay for cheap and now have 4.  2 tiny PSB sub series 100s and 2 Episode 8" Evo subs.  Episode is a custom install brand and has terrible resale value because nobody knows what they are.  I took a small chance for about $150 including shipping and it's definitely better than I'd expect for that price.  They're not blow the doors off HT subs but get you down to the high 30s.  All 4 subs are sealed.  

I dunno... I might not be popular here in this discussion, but oh well here goes:  I went for many years without a sub at all, with my Epi 100s...  never felt the need.  Have one now, though, and do like it, but it wouldn't be the end of the world if I didn't have one at all. 

Maybe it'd be worth the effort if I have time, space, money, and the correct tech and meters and whatnot to balance it all out with two subs, but I'll cross that bridge if I even feel like it when the time comes.  Maybe if I have a bigger room or something, or just if curiosity gets the better of me.  I'm quite satisfied as is, both with my Epi and my Heresy each w one sub in different rooms.

I do like the one dual-side firing sub that I have with my Heresy IV, positioned right between the speakers... and maybe that two-sided firing helps with the "balance" in my room, but I never localize the sub w my ears... it is a complete disappearing act.

@curtdr, Honestly subs are more for movies or just messing with.  They do make a difference with some music but if you mostly listen to music that doesn't have much below 40 hz you don't need them.  If you want good bass the most important thing is to live in a lightly built wood frame house that lets a lot of the bass escape through the walls.