Sub build issues.

Let me first apologize if these questions have been asked elsewhere, but after looking for a half-hour and not really being sure what to look for, I gave up.

I feebly attempted to build a DIY sub based on the Triska Sub project highlighted on Parts Express. My intent was to use this either in my computer setup with some decent powered monitors, or in our smallish bedroom or living room. Anyway, I hastily cut the pieces and threw together a box not-quite-exactly like the one highlighted (I blame my lack of wood-working experience). It's a little bigger. Everything's in, and it has some very decent slam and probably reaches mid-30hz before it becomes useless. Not the best low-frequency excursion, but decent enough for my first build. And at less than $100 for everything.

The problem is that I can hear sounds that I'm pretty sure are above the cutoff point (crossover). Meaning...if I don't have any speakers hooked up at all and just the sub, I can pretty easily make out the vocals of the song, and I'm pretty sure I'm not supposed to be able to. It becomes more apparent the higher the crossover point, but it's present regardless of where I have that set to.

Is this an enclosure thing? I did get the acoustifoam, but would say I was pretty stingy with the placement. What is the general rule of thumb when it comes to the absorption materials? Or is it a driver thing? I'm using the
DAYTON SD215-88 8" SHIELDED
DVC SUBWOOFER
as the powered woofer, and
DAYTON SD215-PR 8"
PASSIVE RADIATOR
as the 2 radiators. Or could it be an amp issue?? I have the DAYTON SA70 70W installed.

I'm leaning towards a build issue, but an unsure where to begin looking for a fix. I still have enough MDF that I could probably re-make the enclosure (which I probably should do...the dimensions are totally off, and there's even a small point at which the front doesn't meet up with the side, but other than some air movement noise I'm not convinced that's the culprit).

Any help would be fantastic.

The sloping rolloff makes sense, there just seems to be less of a rolloff than I imagined. But thanks, I will work harder at integrating and see where that gets me.

Maybe I'm thinking of one thing and calling it something completely different. After a little bit of listening, I think what I'm experiencing has less to do with the crossover sloping effect, and maybe more to do with...hell, I don't know what it's called, but certain frequencies and/or noises seem to linger longer than they're supposed to, almost like they're echoing inside or something. I believe that's what made my think it might be a crossover issue. I was listening, and it was audible above the sounds of the mains and distracting. Would that be an issue with too little acoustic suppression foam and the like?