First of all I would skip the XPA-3 and put that money into your speaker and subwoofer budget. If you don't feel like your Yamaha can power your speakers to adequate loudness levels, only then buy an external amp, but, in all likelyhood, the Yamaha will be more than enough. Note that the Emotiva will only get an approximately 3 dB gain over the Yamaha anyway, so it's really just not worth it.
If you want better control over your bass, get a multiple subs setup with some optimal placement. I know that sounds counter-intuitive, but a single sub is far more likely to produce a poor response and give you boomy bass. Your room is going to affect the sub's frequency response, it is going to create peaks and nulls in the bass. Multiple subs can help shore up the nulls while equalization can temper the peaks. It will give you a smoother and more even response throughout the room. Especially in the case of the subs you are looking at, none of which are very powerful, and are not appropriate for an open room of that size. The subs I would be looking at are the
Hsu VTF2 mk4 and the
Rythmik FV12r, both of which will outperform the subs you are considering and cost about the same or less. They are bigger than the sealed subs you are looking at, but they are a lot more efficient and will have much better bass extension. Don't think that because they are ported that they will sound sloppy either, they will match the sealed subs for sound quality as well.
One more thing, it does take some experimentation with placement to get the best bass, so hopefully you have different spots where you can place the subs. This will have probably have the biggest effect on the sound quality of the bass.
