My first impression is that the prior owner TURNED DOWN THE OUTPUT OF THE SUB VIA THE 5.1 or 7.1 channel Equalizer.
If your not aware... you have a built-in adjustable equalizer to increase of decrease the LEVEL of each of your 5.1 or 7.1 channels, using the Microphone provided by DENON.......
Read the manual and it will walk you through how to adjust the outputs, using the "BUILT-IN signal generator. ( Not knowing how large and the crossover point of your "Best- Bullsh-t" speakers, I suggest setting the levels to crossover at 80hz....... Then let your SUBWOOFER do the remaining work.
I also suggest you place the Mic on a Camera stand isolated ( away) from any reflecting walls or hard surfaces.
If you have a DVD player, buy a cheap home theater set-uo disc and use that in the interim to get some semblance of channel balance. These will run through the channels one by one and provide a visual level to roughly set levels....Better than nothing at all.
AS for jumping the gun and claiming a defective product....... hold off until you perform the steps I provided. Then, you will know for sure..... Good luck. Be patient!
Anyone else have some to add?