I suspect as others have indicated you are clipping the Subwoofer. But not the Subwoofer amps, rather you are reaching the excursion limit of that small Sub.
I suspect you need minimum a good 10" high excursion Subwoofer.
Also, you haven't given us any framework within which to make a judgement. How loud is loud. Assuming your AVR is calibrated, what volume level are you playing at.
Also, how is the amp setup. A vast majority of people will set their Front/Center speakers to SMALL, which diverts low bass away from them. That means less excursion and more clarity.
If we assume you are delivering 1 watt to the speakers average to get an output of 86db, which is just a trace above Reference Levels, then this is how the power breaks down as the volume increases -
86db = 1 watt
89db = 2w
92db = 4w
95db = 8w
98db = 16w
101db = 32w
104db = 64w
The standard Reference Level is 85db with 20db headroom for 105 db peaks. You seem to have the power to achieve those levels from your front speakers. Though most people find these levels extremely loud.
If your AV Receiver (Pre/Pro) is calibrated, then the above Reference Levels would be 0db.
Lastly small subs do not go that deep. I think the Wharfedale 10.SX has a rated low end of only 35hz. I think if I tried real hard, I could find a Bookshelf speaker that goes that low. The Diamond 10.2 are rated at 40hz at -6db.
Generally to get deep bass and to push enough air to feel the impact of the bass, you need minimum at 10" Subwoofer, and in a large room, you need minimum a 12" Subwoofer. How far you want to take it from there is up to you.
Generally manufacturer Subwoofers are not as good as Subs from people who specialize in Subwoofers, like SVS, Velodyne, REL, and similar.
My listen area is 16ft x 17ft, though open floorplan into 17ft x 30ft. For music I use speakers that have 2x8" woofers each. For movies I add two 12" 3-way to the two 2x8" 3.5-way speakers. No Sub, but I move some serious air for some serious impact. Those speakers do not overwhelm my space.
My point is, given that your space is 15ft x 20ft, and on the assumption that the TV and sound system are oriented on and toward the narrow wall, larger speakers would not be amiss in your space.
The amps are not overly powerful, but they are adequately powerful as I have illustrated above.
The Sub goes neither all that deep or all that loud. If you have a weak link, it is likely the subwoofer.
Though again, as others have pointed out, we don't know how loud you are trying to play.