Car subs in my house??

the amplifier to power that tiny 6" is not nearly powerful enough to power a real subwoofer.

amplifier is probably wrecked too

Is the impedance between the old Klipsch sub and the new one the same? Also, are you trying to run a pair of Kickers from the same amplifier output that was powering the single Klipsch sub?

You might try getting a generic 6" midbass from somewhere like Parts Express and trying that. Make sure the impedance of the new sub matches up with the original one.

Car subs are made to operate in 'rooms' averaging 100 cubic feet. Even a small living room is about 1000 cubic feet. If there wasn't a major difference in how they work there would be no such thing as auto sound versus home audio.

The T/S specs will be somewhat different given the conditions they're put in, but the basic operation of both is the same, no?

Impedance loads vary based on the differences in home vs mobile amps and the necessary output from each sub to match its operating environment (car subs will be built for significantly higher cone travel due to the necessity of moving a large amount of air to compensate for the much-increased environmental noise in a car vs a house), but I don't think that would really cause the issues seen in this case. Plenty of guys ran Adire subs in car audio that were designed for home audio and had spectacular (significantly better than JL's competitive designs at the time) results with that.

I can see output and FR being totally borked in this cas, but I wouldn't think the static/crackling noise would be due to the "type" of subwoofer used, right?

The basic operation is the same, but so is that of a Piper Cub and an F22.
If you look at the SPL chart here the difference between response in a car and out of a car is huge:
http://www.jbl.com/resources/Brands/jbl/Products/ProductRelatedDocuments/en-US/BoxesandParameters/GTO1014TD.pdf

That doesn't mean you can't use an auto sound driver in an enclosure better suited for the home, but the better the driver works in an enclosure designed for a car the worse it will work in an enclosure designed for the home, and that's when you put the driver in an enclosure designed for the home, which the OP has not done.

Right, I understand about enclosure design as I've done a few of my own; I just wouldn't think taking a car setup and running it in a house would cause the crackling noise the OP was referencing. I've run subs free-air, sealed, ported, 4th and 6th order bandpass, and in snailshell and ABC boxes with power ranging from significantly too low to significantly too high and never had issues with signal degradation. The FR was quite borked, but that was it. I know HT is an entirely different environment, but physics stays the same.

I'd be more inclined to attribute the signal issues with the amplifier, rather than the subwoofers and enclosure. I'd completely agree that the "sounding bad at high levels" would be directly related to the drivers and enclosure, like you said.

Looking at that chart, isn't it generally going to be true that the larger the space volume you have, the more difficult it will be to fill in the low end? A 10" sub might get you 20Hz in a Civic, an 18" might do it in my living room, but neither is going to do much in an arena.

I'm not sure what your point is about the enclosure. The OP has the car subs in a car enclosure, so what? He's not trying to hit THX specs, he's trying to work with some computer speakers. Assuming the amp worked with the subs (big if), it would probably sound better than the little sub he's replacing.

Yes. That's why HT systems can reach to 20Hz and lower at high volume, while real theaters don't even try, usually high-passing around 30Hz.
The larger the room you have the larger the enclosure required to offset the reduced cabin gain.

peaky response will be attributed to car box used in a home environment.

crackly sound would not be. As some else said - the amp is toast for the sub, or severly underpowered, or there is an ohm mismatch and the computer speaker amp isn't designed to run the lower impedance your car subs are using.

Just for fun, I've tried to use a myriad of car audio subs in my home theater room to mostly poor result as Bill has suggested, however, never did I hear a crackly sound on any of them. I am actually using amplification designed for the task however.