Are you running the 6x9 with full pass or mid??
Sounds weird to me as to why the spendy speaker wouldn't go lower, mabey the speakers specs didn't tune into that box. I do know that some speakers have to be within tolerance especially for a band-pass box. High roll or extended movement subs don't like band-pass boxes to much as they are tuned for the extention, speakers that yield good responses at a smaller extentions like sealed boxes and most times require the least amount of volume for a good sound. Also I had a few experiences where extended movement subs actually destroyied band pass boxes.
I need new 6x9's as they distort because the subs are right under them, they are some sony ones that push mids like crap. Subs face up, ports pointing towards rear.
When I got my subs I had to look up the QTS and such to figure out port length, putting some poly fill helped balance the volume in the box. The alum10 didn't like to be sealed at 1.2(1 cubic with speaker) cubic so I ported it.
I have always liked Infinity's perfect kappa series subs and components, I once had two 12inch, two 10's perfect kappas, and 5 kicker amps, and reference series comps. My rear window blew out once and my front windshield cracked from my old system. That system rocked as did many of my friends systems.
My old school friend ran those Old cerwin vaga adjustible cone subwoofer-speakers strokers, man was it $%^ loud, he ran 4000 watts RMS and could feel the ground literally vibrate. **broken link removed**
Oh and the reason I run mine at 6ohms is because my car can't handle the current draw right now as the same alt has been in it for a long time and has been abused. I am now a poor school guy that has no money lol, I had to sell alot of my stuff but held onto a few amps.