A fellow Model S owner told me in discussion that you can remove the factory premium/UHFS subwoofer and replace it with a SoundQubed HDS2 subwoofer, add an amp, and there is a significant improvement in sound quality.
I have not looked into it further than that, but maybe another options. I know the sub is less than $100 bucks, then amp. So definitely more cost effective than the NVX sub/enclosure.