Subwoofer Reverberating on Certain Bass Tones

My new subwoofer is providing a strange reverberation or "wub-wub" sound on certain portions of deep bass in certain pieces of content. Hoping to see if I can get some advice as to whether its a defect, or whether the base is just too deep for my system to adequately handle.

I've got a not so fancy 5.1 system (Monoprice 5.1 Product #9774, the relabeled version of the Energy Take Classic 5.1 from what I understand). I know it's not a top of the line speaker system, but I thought it would suit my needs in a second room and the last cheapy 5.1 HTIB setup I got (the Harmon Kardon HKTS-15) serviced me fairly well. I'm not an audiophile, but I like a quality sound and the speakers on this system sound great to my ears. The sub on the other hand is giving me issues. This system is hooked up to an Onkyo TX-NR616.

I first hooked it up and let it age with some music/TV for a couple of hours while I was at work. Then I went straight to gaming and played a quick cross-section of some of the games I'm currently playing (Halo 4, Dead Space 3, Assassin's Creed 3) and the sub seemed to handle things quite well. It isn't incredible, but it was good and a step up from the HKTS-15 I had been playing on. It sounded fine with episodes of Walking Dead Season 1 on Blu-ray, including the first episode with the speeding cars and the car wreck near the start.

Then my wife and I went to watch Game of Thrones. Immediately during the opening credits sequence the deep bass of the intro-music started to cause the sub to "wub-wub" along with the music - not all the time, but definitely frequently throughout the intro-music. When my wife notices an audio issue, I know its really prevalent. So I investigated and flip the sub on its side and could see that as the bottom cone was pumping it was producing the reverberation in tandem with the vibration of the cone. The reverberation is really loud, and it was clearly audible over the intro-music coming from the satellite speakers.

The Onkyo 2EQ setup in the room had adjusted the subwoofer to -7 already. I checked the master volume of the sub and found that it was set to 75% so lowered it to 50%. That decreased the reverberating "wub-wub" but didn't eliminate it, and it was still very audible over the rest of the music. I lowered the sub volume to where the reverb was gone (about 25-30%), but then it seemed as it from the normal viewing position you could barely perceive any bass at all, as if it was off almost, so that is not a solution for my preferences. I also played with the low pass filter on the sub itself but it did nothing, probably because this bass tone is so deep already. I have not messed with the crossover settings on the Onkyo except that I ran the 2EQ setup.

Being that it was late and my wife just wanted to watch TV, I set the subwoofer back to about 50-60% and we proceeded to watch. Maybe once throughout the entire episode we noticed the reverb. But as soon as the episode ended and there was a dramatic bass tone and then the end-credits musics we started hearing it again.

We watched all of these Game of Thrones Blu-rays on our older HKTS-15 (in a different room of the house) and never had this issue, even with the sub up pretty high (the HKTS-15 sub is 100W and the Monoprice sub is 200W). When I go home tonight I'm going to try some other content to see how prevalent it is.

Is this just a product of a cheaper subwoofer not being able to reproduce really low bass? If so, why did my older and less powerful HKTS-15 subwoofer not have a problem with it? Or could this be a defect where the cone or something else is too loose and I should call Monoprice for a replacement.

Someone mentioned poly-fill in another thread I was reading, and they said that it seemed to abate much of the issue. I'd be willing to try that. Once I open up the sub, where should the poly-fill go? And how thick of a layer of poly-fill should I use?

Thanks in advance for any help that anyone can provide?

I'm assuming you mean that I'm correct that the cheap subwoofer is just struggling. That said, are there ways to minimize this issue with poly-fill or should I just start saving for a new subwoofer?

If it weren't so distracting and something I had never dealt with on my other cheap subwoofers, I could live with it. As it is, I hear it and it drives me nuts especially since the next season of Game of Thrones is going to be starting up soon.

As an additional point, this particular subwoofer has a Left/Right input, although I'm typically used to utilizing a single subwoofer cable to run from the single output on the Onkyo 616 to the single input on the subwoofer.

In this case, the instruction manual is saying to connect the single subwoofer cable to the Left input, leaving the right input open.

Monoprice Technical Support is suggesting I will get deeper bass and possibly correct the reverberation issue by utilizing a Y-splitter to get a Left/Right input to feed into the subwoofer. As to the latter, he suggests the sub is producing the "wub-wub" reverb because it is not receiving a proper signal from both the left and right.

Is there any truth to either of those statements (better bass and/or a fix to this issue), or is Monoprice just trying to sell me a cable/this particular tech support guy is lousy?

Yeah, I know it's not a top of the line system. I kind of thought this qualified as an entry-level system though. I'm not an audiophile nor looking for audiophile quality. I guess I'm confused why CNET would give this such a quality review (they admit you'll get better for a bigger purchase) but both this and the Energy Take Classic that it's a repackage of have no mention of providing awful reverb noises when in use in their review. I know for more money I'd get something that was crisper, deeper, richer, etc, but this isn't just a lack of ooomph in the sound quality, it's pretty much a deal-breaking noise when watching something which I'd expect to be noted in a review. Nor do user reviews from any site mention a similar problem. The room its in isn't that big, and the seating area isn't very far from the sub/speakers.

The sub input is a Left/Right line-level input. The instructions provided said when using something like the Onkyo with a single Sub-Out, to connect the single sub cable to the Left input, leaving the right input empty.

With that said, the Monoprice support team has suggested that if I used a Y-Splitter to split to a Left/Right input into the sub, I might 1) get better bass and 2) eliminate this problem because it might be caused by the sub looking for a signal from the Right input.

Is there any truth to this? Or am I dealing with either a lousy tech support guy or someone trying to sell another cable.

EDIT: Sorry, I didn't realize the first post I wrote asking the question about Left/Right had actually been posted to the site, I thought I'd cancelled out when I saw there was another response. Sorry for the double post.

Thanks, that's what I thought, just wanted confirmation from someone with more experience.

I really appreciate you taking the time to drop some truth bombs and experience on me. Some additional questions then:

1. Is what I am describing "Port Chuffing"? I'm still really new to home audio and just took what I got with my first system described above. If not, is there a better technical term for it when seeking further help/return options from Monoprice?

2. You mentioned the driver size. Would I be better off with a 12" sub powered at 150 watts in addressing this issue? Monoprice offers one for slightly less than they offer the 8" so I might be able to swap with them (they already told me they can't send a replacement of the 8" I currently have if it was a defect because they are out of stock). I realize this too is a significantly budget alternative, but the driver is larger. Or, is the size of the driver irrelevant to this issue when we're still talking budget subs? Link to the specific model below.
http://www.monoprice.com/products/product.asp?c_id=109&cp_id=10906&cs_id=1090602&p_id=9723&seq=1&format=2

Thanks again.