You really need to experiment with sub settings and location. You can first try setting the subs crossover to be about 10-20 Hz higher than the low frequency response of your speakers. This allows for a nice blend of the Bass from the tower speakers to me more homogenized with the subs. Of course, you will need to play around with slope, phase, etc., and just listening to get the sound right. Other methods, the one you read about, would IMO prevent the sub from doing much work at all, unless you want to hear 20hz organ music and other very low frequency notes. This would entail a lot of subwoofer processing power, size, etc. depending on room dimensions to get this aspect correct for that purpose only, not really playing any of the bass associated with the range of 30-50 hz where many speakers would need a slight boost in bass.
I use a Velodyne SMS1 EQ which I use to measure the in-room bass response with or without the subs turned on. It’s really handy to see what impact the adjustments have on the bass sound because you can see on the monitor the curve (15-200hz) and how adjusting each parameter will either smooth or cause jagged peaks. This Velodyne allows one to also adjust slope, rollover, and cutoff, phase, etc. As others also have stated, two or more subs are better than one. One thing I notice is that if you increase the bass volume too much, the soundstage collapses due to bass smearing. This is the effect of the powered bass not being perfectly time aligned with main speaker drivers. That’s why you need to experiment. You should not hear bass increases in reality, it should be a smooth transition which provides a congenial in room response. For ultra loud and excessive bass emphasis, like rock concerts volumes, forget what is stated above, and adjust the subs to just play loud because at such extreme volume levels no one cares about sound stage, just loudness and bass.
Listen to music with drums, standup bass with and without the subs, and also listen for collapsing of the soundstage with subs in use.
