Gene DellaSala had this to say about inexpensive tSc subs:
"Although the ASW-8 is bland in styling, because of its smallish size, it doesn’t draw much attention to itself, at least until it's fired up. Make no mistake however that this little sub is much more capable than one would expect given its size and price. If I were on the market for a $300 subwoofer, I’d pocket $100 and buy two ASW-8s."
This is from the second paragraph of the conclusion:
http://www.audioholics.com/reviews/speakers/satellite/tsat-1000-ht8-b/tsat-1000-ht8-b-conclusion
To quote Gene again, he says:
"In almost all circumstances, installing multiple subwoofers in your theater room will yield significantly better and smoother bass response across a wider listening area due to modal averaging."
Taken from a comparison of 2 Axiom EP400's vs 1 EP500:
http://www.audioholics.com/tweaks/get-good-bass/two-subs
You did not say how you would use them, but I believe the benefits of dual subs are better suited to music than to HT. For HT, the sacrifice of smooth bass response of duals to get lower frequencies of a more capable single may be beneficial. You're not likely to notice a drop-out or spike in the frequency response during an explosion - as long as it is deep and loud.
Matt is a good guy and that is a good deal on the PA-120's.
However if you like to determine things for yourself, and are willing to float some money on your credit card, you might consider buying two ASW-10's ($240) vs a T100 ($200) or T300 ($300).
tSc has their subs on sale and offers free shipping both ways. Although you can get the ASW-8 Gene mentioned above for $80 each, I'd have to go with the ASW-10 for $120 to get the bigger drivers and amps.
Good luck.
http://www.thespeakercompany.com/Subwoofers-C17.aspx