I belive you saw the digital stereophony comparison there.

Klipsch promedia 2.1 is an okay setup if you are happy with the sound. I'd go with brighter sounding setups from Edifier if I were to choose. (perhaps the C3X, S530D, S350DB)
But if tight bass then nothing beats a pair of dedicated bookshelves. (like the R2000DB or if can the S1000DB for speakers that I can easily recommend)
The Behringers are a decent pair if you are good with studio flat sounding speakers

None of the ones I listed are any bassier than the promedia 2.1 as that has more headroom in bass volume but I always find the promedia subwoofer sounded like trying too hard to produce volumes it couldn't do. Tons of port chuffing on heavy bass and pushing the volume a tad more than nominal and you can hear it distort before any solid bass is produced. Afterall I find the overall setup of Promedia 2.1 very dark and muted in the highs.

As to how are you going to pair a subwoofer in the future? I do have a method that might cost a lil more money.
I bought a dedicated DAC usb soundcard amp that has a volume control to control my master, split the output and send one to the sub and one to the fullranges.
I'll get the volume balanced out between my sub and fullrange and ignore both sub and fullrange volume and only using my DAC to control the volume.
If you're going through a PC and your keyboard has multimedia volume control, those works too. It'll save you some bucks
I'm using a Gigabyte K6800 keyboard with a rotary volume control for fast volume up downs

Depending on the subwoofer you purchase, some has an rca stereo line out bypass where you can chain your subs in between. Like DAC>subwoofer>fullrange
A much cleaner and better setup I'd say.