I put 2 pillows from walmart into my subwoofer box. It pretty much filled half of the box though. It's a 3 CU FT Box and it feels the sub isn't as articulate as it could be.
I also used the NU4-6000 to power it and am only using one channel of it with a Dayton Ultimax 15.
Should I remove a pillow and perhaps bridge it and use 2 channels instead?
you have a good driver/enclosure/amp. no problems there. if you are listening indoors, what is messing up your response in frequency and time is your room. pillows, driver, and amp are the least of your concerns. take some measurements. see what your problems are. tackle them. then it will all be clean and articulate. also ensure that you have good integration with the mains.