Hello scm,
Excellent, from what I've read, the HSU ULS-15 mk2 are a very good choice.
I suggest you first setup and audition a custom 3-sub DBA system in your room, using your 15" Revel/with Dayton amp and your 2 15" HSU mk2 self-amplified subs.
It's important to position each sub optimally in your room. I think using the crawl method, starting your search at the right front corner of your room and moving counter-clockwise around the perimeter of your room, should be sufficient for positioning each sub but you're welcome to use a more sophisticated method if you have the equipment and know how to use it.
Once all 3 subs have been optimally positioned, proper setting of each sub's volume, crossover frequency and phase controls are the next critical steps. Ideally, your goal is to have the volume of the subs match the volume of your main speakers and resist the urge to turn their volume up too high, you only want the subs to be active when the musical content demands it. I've found it useful to set the volume as low as possible with the bass still sounding powerful, dynamic, detailed and natural.
For the cutoff frequency settings, I suggest running your main speakers full-range and initially setting this control on each sub at about the rated bass extension frequency limit of your main speakers. You can then adjust this setting slightly upward and downward until each sub is perceived as seamlessly integrated with your main speakers.
For the phase settings, I suggest you initially just set each to the position the bass sounds best to you. I don't think this setting is as critical as the other 2 but I can detail more precise methods of optimally setting the phase on each sub if you'd like.
I'm more than willing to further assist you when your new subs arrive if you'd like. Just let me know on this thread or PM me.
Tim