The AV-8 has a very nasty bump at 2kHz which is difficult to tame. It is quite audible, especially since Adire's designs cross to the tweeter right on top of it.
For low-end extension and quality midrange, you'd want a ScanSpeak woofer. If they're worked right, they sound wonderful. But you can't get 5 speakers for $800.
Dennis Murphy has a design using the Usher woofer that should be pretty good, but that still might be out of your price range.
I would definitely suggest a kit - it takes several months of hard work to learn to design a pair of speakers from scratch.
I have never heard Danny's designs with the GR's, but having used the M130 woofer, I am impressed by it. Its only fault is that it doesn't move much air. From what I've read about his designs, they are very good. And this is from other designers, not reviewers. So even though I've never heard them, I am comfortable recommending them.
I've only used the woofers in sealed boxes, which I prefer. Sealed, they will roll off at about 85Hz, which would blend almost perfectly with a subwoofer.
Other designs I recommend you look at are Dennis Murphy's and Rick Craig's (
www.selahaudio.com - you'll have to e-mail him). Both have an excellent reputation. If you have more budget, Geoff Dillon's designs are highly acclaimed (
www.dillonacoustics.com)
And, of course, I could also recommend my speakers, the Models 1, 2, and 3 (in that order). I don't normally push them on people, because as far as I know, I'm the only one who's heard them.
What has been the maximum low end that the GR woofers have give you? How do the woofers compare looking at output?