I bought the finest two speakers i have ever owned for my computer, incredible improvement in listening pleasure. But . . .?

They call them subwoofers. Some of them come with their own amplifiers. Basically  a crossover (a circuit board) sends the bass frequencies (about 150 Hz and lower) only to the subwoofers, while the main speakers continue to reproduce the treble and middle frequencies (20KHz to 150 Hz). Since bass frequencies are non-directional, the subwoofers cane be put just about anywhere and you may only need one of them. In fact, many manufacturers only market subwoofers with a single woofer.  Subwoofers became popular long ago, before computers, when many people want to keep their main speakers small but want to add bass response to their stereo systems. Luckily for me, I have some Infinity home speakers that I bought in the 1980s that put out very good, tight bass, so  I do not need subwoofers. In fact, because my speakers have a total of 4 10" woofers compared to a single 10" woofer in many "subwoofers" sold, I bet my speakers can produce deeper bass than these subwoofers. 

What you want are subwoofers that have a flat frequency response, If they do, then they would be matched to your main speakers. Of course you should be able to adjust the sound level produced by the subwoofers so that they are not noticeably much louder than your main speakers. 

The Yamaha YST-MSW10 would be suitable, they connect "inline" with the output from the PC and the stereo speakers plug in to the sub.

I don't think they are made anymore, but you can get them from ebay etc.

They are an active-feedback (servo) powered subwoofer and quite powerful; I've had one under my computer desk for at least ten years.

They are sometimes paired with some Yamaha active speakers, but will work equally well with any active PC speakers.