Your experience of moving to two subs

Yes group delay matters a lot with subs. group delay and phase are related. The audioholics information is very good.

Many subs are delayed enough they are a full cycle back. The kicker is it can change at different frequencies so you can’t just adjust it out with a simple phase knob. At the lower frequency the cycles are so long (slow) it is less of an issue but still very relevant. It is one of the down side of having all the high tech processing in subs these days. It all takes time even though it is still pretty fast. I personally will not buy a sub unless see the group delay measurement as I feel it is important. In a system where timing can not be corrected (99.9% of 2ch preamps). Think of it as input lag ratings of computer monitors.

Gladmo,

you might be hearing a longer group delay that could make it sound “slow” I have not seen the SB2000 group delay but I was a bit surprised at how high the group delay was on the SB 16 Ultra (link below). It was over 20ms.  You could argue at that low a frequency it is less important but still about twice what JL does with their subs. The only way to overcome the delay is using an active system to delay the mains. Receivers can do this and same fancy highnesses as well.  Well a mini DSP does to but the output stage is junk.  anyway just a guess and thinking out loud. 
 

https://www.audioholics.com/subwoofer-reviews/svs-sb16-ultra/measurements

I had one sub broken for awhile (I normally run two) and when I had one I had to really mess around with placement, crossover point etc. got the other sub back so back to two and the System just snapped right back into place. Sounded even and right again. All I can figure is only using one was making lumps in the frequency response and sometimes it sounded fine and other not so much.

now 28hz is pretty low and might not benefit as much. I was running 60hz at the time notes above.