To get a sub to integrate well with speakers. There are two things that have to match in my humble opinion :
1. Both the sub and speakers need to have the same tone of bass at the cross over frequency. Usually the 5.1 speaker packages designed by audio companies work a charm. Because the manufacturer, has considered the ideal cross over frequency range likely to be used, and will tune the included sub to have the same tone at those similar cross over frequencies. So the transition of bass from speaker to sub woofer is seamless. And one cannot make out by listening alone if the bass is still coming from main speakers or the sub.
2. Both need to have the same speed. Usually warm sounding speakers are made warm sounding by having a slower bass response. These speakers ideally match in speed with subs of driver sizes of 12 inches and above. Forward sounding speakers usually have small mid range drivers and are fast sounding. These ideally need subs of 10 inches or even 8 inches driver size to match in speed. If I were to consider adding a sub for music to my main speakers having a 7 inch bass driver, I would consider subs with twin 8 inch driver size and not a sub with a single 15 inch driver.
3. This third point is the most complex. Not many amps come with the flexibility to properly integrate a sub. Most are just two speaker out binding posts with a sub out, which is basically a full range pre out. So despite adding a sub to these pre-outs, the main speakers are still receiving a full range signal. So the bass is being provided by both the speakers and the sub. And unless point 1 and 2 above are achieved, otherwise the bass will still sound like coming from two different sources.
So ideally a amp with low pass limited is required and there are not many on the market with that facility, with the parasound halo Integrated being an exception.
A well integrated 2.1 system will anyway blow away a pure 2.0 system. But all the 3 above points have to be satisfied to get that integration. Thanks