@jallen, I suggest you take a look at SVS which IMO offers great value and in addition to variable phase provides the ability to adjust level, room gain settings, PEQ and XO frequency and slope and more including the ability to save presets. Being able to set the slope to match that of your main speakers eg. 6 -24dB/octave improves phase tracking. All this can be managed on the fly with your phone using the SVS free app. from your listening position. There are other brands that do similar but I'm not sure if any of them offer the remote adjustment feature.
What I've not yet mentioned is that variable phase essentially allows you to place the subs where you would like them to be, where they are unobtrusive or visually attractive. Unlike phase flip dinosaurs.
If you do decide to use subs choose the easier to integrate sealed box designs which to my ear are superior. Also to mention that 2 smaller and cheaper subs will always outperform a bigger single sub by providing a smoother room response. Set-up is best accomplished by measurement and in addition to reading papers from the 2 gentlemen mentioned in my previous post there are hords of YouTube videos on how to accomplish this. It's fascinating, inexpensive and fun. Anybody not making use of properly dialed in multi-subs with measurement is seriously missing out. It's not just bass that improves, it is across the entire spectrum.
Unfortunately for some, those who don't turn to science, are content to believe a forum post or some salesman, of dubious knowledge and experience, who claims: "Sir with these fantastic new fullrange speakers you will never need subs" So sad 