Once again this is not for bass heads, this unit works just fine for lost of customers out there. The infiniy basslink is better however much bigger. In fact that's at the boarder line of just running a zenclosure and a normal sub with the space it takes up. It's no where near as compact as the kenwood. As far as the 40 watt comment, i have seen a normal sub hit good with a high efficiency rating with 50 watts.

If a 5x7 ( the size of the kenwood "sub") is giving you what your 6 1/2 can't, you need some different 6 1/2's.
This unit is laughable, at best.

The %x7 has a bigger surface area than the 6.5 and was also designed to produce lower frequencies where as your speakers should not be crossed that low.

Now you're splitting hairs. 5x7 area 35 sq in..... 6 1/2 area 33.2 sq in 1.8 sq in difference. Alot of manufacturers are actually using 6 3/4" drivers nowadays, which would put you at 35.8 sq. in hell, thats bigger than that "sub"
As far as the crossover comment goes, I dont exactly know where to begin. I'll crossover my woofers as low as I like, thank you very much... Components do not have a high pass on the woofer. The woofer plays from its lowest capability up to a fixed crossover point, where the tweeter takes over. The woofer is basically a full range speaker. You can certainly chose to place a high pass on your woofer if you are running an active system, but the majority of folks do not get that far into these things.
That being said, The kenwood 5x7 is rated to play to 35hz. The Polk MM 6501 components that you recommend to many members play to 40hz. other components are rated even lower.
And I would bet my hat that the kenwood rating is, shall we say, "tweaked" for marketing purposes. I'm just calling a spade a spade.

Your splitting hairs as well. If you want a lot of bass this is not the sub for you. End of story. If your looking for something to help fill in the low end this is perfect. I am not telling you were to crossover your speakers, I just know that if one woofer has to do below 60-80 than your trying to make it do to much. Where as using a sub to take care of those would help your woofer be more efficient. The kenwood will still play lower than the polks you said that yourself. There is always something better. Coaxial speakers< Component speakers<Compact small powered sub< Full size self powered sub<Sub in box with external amp< 10 inch < 12 inch< 15 < and so on. That's probably one reason you went and built your own sub box using a external amp, simply because you wanted more bass and the ability to even go lower in the Hz range.

http://www.the370z.com/audio-video/6...box-today.html