Slopes, X-Overs and Bass

I've never gotten the kind of production I felt I should be getting from the subchannel on my amp and/or my sub.  The other day I was playing around with the x-overs on my head unit's sub out and noticed something interesting.   Going from a 24db slope to a 18db, 12db, 6db, and full pass was like slowly turning the volume down on my sub.  In other words, I was getting pretty good production out of the sub with the 24db slope (regardless of the cutoff frequency), but there was a noticable drop off every time I lowered the slope.

The subchanel on my amp has a fixed low pass 12db filter in place.  I can change the cutoff frequency, but I can't turn the filter off and run full pass (allowing my HU to handle the x-overs).  Since I can't turn it off, I turned it all the way up to 200hz.

What I don't understand is why are the higher slopes making such a big difference?   I thought this might be a phasing issue, but I notice the same difference in production with the front and rear spakers disconnected.  I talking with someone at another forum the other day, and if I understood him correctly, he though this was because the amp's crossovers are inferrior to the HUs and that the higher the slope I used, the more I was "defeating" the amp's built in crossover by sending it less signal to work with (or possibly mangle).

That sounds reasonable to me, but I just wanted to get some other opinions.  If it is the x-over on the amp that's causing me so many problems, it may be time to get a new amp, or at least a new sub amp.

Always, man.  Always. 

I hadn't updated my sig line when I posted this, but I took out the VR650 components up front and had a set of CDT EF63-Neo braxials put in and I LOVE them.   I'm not going to rag on the VR650s because I'm sure a large part of my disatisfaction with them was due to the install (tweeters mounted high in the doors and over a foot away from the mids), but I've got very good imaging, a nice soundstage (although it could be a little higher) and I get no listening fatigue whatsover.  Also, since I didn't feel the need to attenuate the tweets 3db or cut the high frequencies on the EQ, I'm getting a much more natural sounding high end.

[QUOTE]You seriosuly need to dump that w1 and get a pair of real subs eh? [/QUOTE]

Hey, that sounds good to me.   What amp/sub combo would you recommend?  I found a local shop that I've very comfortable dealing with, and they sell JL Audio, Eclipse, and Diamond.   Anything in those lines appeal to you?  I've heard a lot of good things about the Diamond D1s that will be out soon. 

[QUOTE]Hmm... perhaps geepherder has the right idea... with a small correction:

Yeah, that makes sense, but I'm still surprised that I would notice such a big difference between slopes.

[QUOTE]What kind of car are you driving?[/QUOTE]

2004 Honda Civic sedan.  Don't let the sedan part fool you though.  It's a still a very small car.

I'll give that a try and see what happens.   If it does turn out to be the car's resonance frequency that's creating the problem, how do you go about correcting something like that?