They do very well on the low end in such a big box. And give me 2-3 weeks and I will have them on 6krms. Just sent the amp back in yesterday 3/23. It should get there by coming thursday. See what they say, and they will either send me another amp or fix the one I sent in.
and about the BL and the HD3 deal. Both are good subs, pick the one that will benefit the most from the amount of power you plan to give it. If its near 3krms, then HD3, if 1k-2krms, then the BL is a great choice. and will save you some $$. SPL Choice is selling this dual 2 15" hd3 for 260 or so, around there. Hit him up about it. But if you want new thats a different story.