I should add that PSA XS15se is also an excellent (tight) sub.
This is the only chart I have seen that rivals the Rythmik.
I am not too quick to recommend it to you because it is a fairly large box 19.5"x17"x18" (HWD), and I assume that is low on the WAF scale.
As you can see, both the Rythmik and the PSA sealed subs are much tighter (and therefore, more musical, IMHO) than the SVS SB13 Ultra (which is a great sub as you heard).
Rythmik does not have free shipping both ways, so it would cost you to audition it.
Tell us where you are (in general terms). If you are near me, you are welcome to come have a listen).
Trust the charts above (measured by Jim Wilson - aka Jman). If you don't want boomy, they tell the tale.
I agree that you may not want a ported sub. If the SB2000 was too boomy, I don't think you really want a ported sub. For example, here is the PB1000:
Some people believe this overhang is not a big deal (not that audible), but your comments about the SB2000 make it clear that you are not one of them!