Hey DarkZ,
I actually have the Polk MM6501 components in my Z right now.
I cannot tell you how they compare to the other kenwood (cuz I haven't heard them).
But I can say that I "think" they sound pretty good. Subjective, of course.
Not having rear-fill (no rear speakers) is still weird to me, but the Polks sound pretty CRISP and LOUD. I definately recommend that you switch the -3db toggle on the cross-over, if you plan to get the Polks.
Speaking of cross-over, they have a "cheapish", "radio-shack-ish" look to them. Yes, you won't be staring at the boxes once you install it, but just kinda has that look. (Oh, and I think radio shack is pretty cool, but kinda "cheap-ish" with some stuff
)
(I recently installed an infinity component set in an altima and those cross-over boxes look pretty high-tech.)
Anywhoo, I own the Polk components, and I think they work nicely.
---AB.