I used to be skeptical of the advantage of a sub with 2 channel audio. Then, a system switch to 300B tubes and some vintage Altec 604 speakers brought a glorious lifelike midrange but a less than satisfying lower bass.
Feeling I was getting closer to my goal soundwise, I thought I'd spend time tweeking further to try and get it closer to an ideal I held in my mind.
First one sub helped, then a second was better. The room is fairly large and it wasn't until a third sub was added at the opposite end that things started to come together. I think the addition of more subs within the larger room environment was crucial as they really don't work very hard individually to load the area.
The subs took a while to get phase,level and crossover correct. Using a radio shack analog meter and careful voicing helped. Also indispensible was that the subs are active but they are also volume and balance controlled seperately from the mains by an SS preamp. This allows full control to the exact level needed by the various music, room character, and degree of operator inebriation.
Further, I found having the subs staggered at various crossover points seems to make for seemless transition. Each sub is a different unit, tho two are Velodynes. Each seems to have a different strength, as one will do better at 20-40 hz and the others seem to sound better crossed higher.
True full frequency speakers with amps of differing character than the 300b units, combined with various room and other parameters will obviously give a different result than in my own. For the last few years I have been fully satisfied with the sound achieved here, even knowing and realizing of certain limitations within the current system.
The system as a whole sounds quite good. The subs are well integrated and you aren't aware of the presence they provide until they are shut down. Then they are quite missed for the foundation they give to the musical content.
My suggestion is to learn all you can by your own experience (with consideration given to the experience of others and on this forum especially) within your own room and system parameters and change as desired until you hit a better combination. Once you feel you are on the right path and getting closer, stop and enjoy what you have. Subtle tweeks within that range might just then take it to the next level.
Cheers,
RW