Given the parallel wiring, about 3ohms from 200-300hz. A stable 4ohm amplifier would be a better option. It's a fantastic design though and should still be considered IMO. Go sealed and crossed to a pair of sealed subs. I'm working on a modified sealed version of the waveguide TMM that's almost complete of the finishing stage. Listening impressions are amazing. With all the great deals on AVRs these days, sounds like a perfect time for an upgrade considering the savings of building the BAMTM!
Your receiver shouldn't have a problem driving them. I built a pair of modified 2.5 cu. ft. BAMTM's that incorporated 8" dvc woofers in the lower sections. Mine were entered in the Iowa DIY2008 (http://home.mchsi.com/~iowadiy2008/index.html) and they placed 4th in the budget catagory. Great bang for the buck design. Have you built them yet?
I could swear there was a chart of it floating around somewhere but I cant seem to find it. They are considered 4 ohm but dip to 3 for a bit. I run them from a hk 347 and have never had any problems. If you are set on an 8 ohm design check out the tritrix here. http://www.geocities.com/cc00541/Tritrix_pg_1.html