I've just ordered the SVS PB10-ISD for my M60s and QS8s. It's in the same price range as the STF-2. I have never heard any sub more than $50, and based my decision entirely on online info such as this forum. The SVS and HSU subs have good reputations, even here where they compete with Axiom's line of subs.
I have to add that both SVS and HSU mostly sell direct, but their websites are so ugly and their online stores are atrociously badly designed. It took me a lot of courage to finally order from SVS yesterday, thinking these guys are sub specialists after all.
Axiom's online store, OTOH, while not super-slick, is quite satisfactory. My QS8 order got shipped in less than 2 hours after I clicked the 'complete order' button.
