Sub Question. One higher quality sub or two lesser quality? (Rythmik vs HSU)

TWO..or more,HSU and SVS both good as I am sure the others mentioned are too.
SVS lets you have free shipping both ways..in case you wanted to send it back,
SVS Also has 1 year trade up policy.Example,
You buy 2 PC2000[Pro version Now but they have the PC2000's at this time]..
and if later in the year you want to go bigger then send them back
and
you get FULL credit towards the Subwoofers that are higher up the price line.'
Free Shipping all the way around.
Power Sound Audio seems to make nice Subwoofers also,
I have a friend that has one and enjoys it.
He[Tom] used to be Part of SVS some years back.

I'm a little disappointed to hear that with two 15" woofers you are even looking for subs. I am considering the CW's. I understand they don't get down to the lowest octave. Not going to derail the subject but could I ask how big your room is? Might have missed it. 

@onhwy61 - Have built OB subs? HAve you used the GR Research servo amp and drivers in a project?


@mijostyn - you and I have mixed interactions, I probably misunderstood the very first question you asked me and the rest was, well...awkward.
I do have questions regarding your post on the previous page, as I am actually like the OP ozzy62, trying to research on this very topic.

HSU all the way. 2 of those tuned properly will be unbeatable for the price. 

definitely one sub of higher quality is any day better than two of lower quality.
in subwoofer more than sheer SPL , integration and matching to the main speaker is a far more important criteria.if you get a better sub the chances  and probability of a seamless integration are far higher