Oh my. :x Not really.
6" subs are more bass modules than subwoofers. You might be able to DIY a little sumn sumn with
this Tang Band W6-1139SIF driver, a 0.4ft³ enclosure and a 1"x5" tube vent. That'll get you -3dB around 36Hz, about 97dB at 30Hz @ 100W, and bottom out around 25Hz.
There aren't a lot of quality 8" subs for cheap either, really. The best one that comes to mind,
Outlaw has the M8 for $300. The
Dayton Sub-800 would be about the cheapest 8" option, but it's most likely going to leave you wanting. I've read some people like the
Pioneer sub, but it doesn't have any advantages over the Dayton as far as I can tell.
Here's an under-couch sub if you've got 4 3/4" clearance under your couch, but I'd be shocked if it sounded any better than the restroom the day after a Taco Bell binge. Well, they offer a 30-day trial (probably minus return shipping I'd guess), so it might be worth an audition if there are no other worthwhile recommendations. Zieg? Any other ideas?
varsis, is your NXG too big to use as an end table?