Well, after this thread a few years ago:
http://basschat.co.uk/topic/175567-which-pa-subwoofer-to-go-for/
I still have the Mackie HD1501s, and they're still working fine!
I really only use for pub and small club gigs,and find that I only ever need one of them to compete against 3k peak worth of tops (RCF NX L24-As), and that's not even driving it hard.
It makes a huge difference when doing sound, as with the best will in the world a bassist is never going to get the balance right all night, and most bass rigs never really dial in the right amount of sub for the room. That being said, I think it's pretty tough for a PA to make bass sound great by itself, and I get best results with a mix of the PA and the rig. I think it also helps to have some of the kick and the bass coming from the same source.
With the full rig I'm up to 5.2k peak, which is probably overkill for most pubs
The 1501s are a complete bugger to move around though if there's stairs, but can just about carry with one hand if needed. And digital mixing is really a must to get the best out of the bass for me.
A.