seemed to be a big hole between where the 6 ½” cut out and the subs cut in.

That depends a great deal on how you set your crossover frequency on the subs.

For reasons I don't understand, most people seem to think that subwoofers are for playing in the 20--80 Hz range, and if they're really brave, they'll go all the way up to 120 Hz.

I hve run my subs as high as 5,000 Hz, or even full range. Due to a system reconfiguration, I can no logner do that -- 250 Hz is the maximum my new sub amp can cross -- and I am not completely happy with the results. I may start running my subs full range and install a passive crossover somewhere around 1500 Hz.

If you are running 10" subs, let them sing. No doubt if you checked with a scope you'd see they were distorting in the higher frequencies, but if you can't detect the distortion with your ears, who cares? [bragging] Distortion or no, just about everyone who ever listened to my stereo has stated without prompting that it was the best they had ever heard, in or out of a car.[/bragging]

If you are running 12" (or larger) subs... well, you shouldn't be if sound quality is important to you.

tanstaafl.