Has2be is right on. The Super Zeros have some nice qualities (I had them in the past, they are not bright), especially how they image and render the human voice. But they have no bass. I too believe your problem is the Yamaha sub, which is not able to integrate well with the quick midbass of the Super Zeros.
For $300 I would recommend looking into used HSU subs with the smaller drivers (ST 2), or better yet, Gallo subs that occasionally come up on the used market. There are, of course, many good choices out there that I'm omitting, but all things being equal, I would try to stick with smaller drivers 6 1/2 - 8 inch when matching subs with speakers that barely go below 100hz.
The second issue, and perhaps the larger one, is successfully integrating the sub with the speakers. Do a search, there are many threads on the issue here on Audiogon.
For $300 I would recommend looking into used HSU subs with the smaller drivers (ST 2), or better yet, Gallo subs that occasionally come up on the used market. There are, of course, many good choices out there that I'm omitting, but all things being equal, I would try to stick with smaller drivers 6 1/2 - 8 inch when matching subs with speakers that barely go below 100hz.
The second issue, and perhaps the larger one, is successfully integrating the sub with the speakers. Do a search, there are many threads on the issue here on Audiogon.