First, the Basstech 7 is rated at 400watts continuous, 800watts peak. A 500 watt amp at 2ohms load is not what I would consider good. I don't want to sound mean, I'm just trying to make a real world recommendation. Switching to something like a Macrotech 2400, where you have one Basstech on each channel would be much healthier, and probably sound a bunch better.
To JimB's comments about not having a lot of punch(kick drum in the 70hz region is how I interpret that), I've not had that problem-I think the fact they have a lower cutoff than many other subs causes the balance of the sub to sound "different", and the lower distortion output probably contributes as well.
By the way, the 400 watt continuous rating is long term, pink noise, 30-100hz. We used to hook them up in the "death room"(and believe me, that was aptly named), and the last time we tested any went 30 hours straight. We stopped because we were tired of hearing it, even with a 30db drop outside of the "death room".
Best regards,
John Halliburton