I can jump in and try to help.
I just received my Volurs yesterday; in all transparency, I just recently sold my U18s' (not because I dind't love them!), so I cannot do a direct A/B, but from very good memory...
First, the comparison is a bit hard because of the driver differences. Second, my thoughts are using the M20 module (which is the only module I used on the 18s).
I like to think of the U18s as having a "warm reference" tuning with top-tier technicalities. The Volur is a bit more of a fun tuning - not at all what I would consider reference. Technicalities are very solid, but perhaps a step behind the 18s as you might expect.
The quanity of bass on the Volur is quite a bit elevated versus the 18s, and there's also a stronger sub-bass emphasis; the 18s' sub-bass rolled off just a tad, whereas I find the Volur's sub-bass has no roll-off at all that I can hear.
The quality of bass is also different as we're talking BA vs DD (not to mention the unique configuation of Volur's DDs). I don't know if I'd say one is better than the other, it's more of a preference thing. I personally have come to appreciate DD bass more (which is part of the reason I sold my 18s)...bit slower decay, more accurate (to me) bass timbre, etc. The Volur's bass has excellent rumble (much more than the 18s as you might expect), I've not perceived any bleed into the mids, and while it hits hard when called upon, I also find it can be quite constrained when the music calls for it.
Hope that helps!