Go dual small sub, switch to closed sub or buy a bigger sub?

Sealed subs aren’t better for music once you hit a certain price range. Also, sealed subs have more distortion than ported; it may be just below audibility for some units, but it’s a fact. 
 
The PB-1000 will be audible down to ~20Hz in-room, so while it’s not accurately reproducing the opening note of Edge of Tomorrow, it covers most content. 
 
Unless you want it louder, not a large benefit in getting a better sub, as the distortion and “tightness” of the PB-1000 is already really good. 
 
Dual subs reduce localization, improve seat-to-seat consistency, and improves linearity (especially when re-running Audyssey again). 
 
So yeah, unless you want your windows to flex, go dual.

If you're listening mostly for music, you might be OK with what you have now.  That is a somewhat small sub for the space, but if it's filling in the bass and you're not trying to flex the walls with dubstep and you like the sound, it's all good. 

What exactly do you feel like you're missing?

Adding another sub can have a number of benefits:

https://www.svsound.com/blogs/svs/75040195-why-go-dual

I'm not familiar with Audyssey let alone their many versions. 

There is no set formula for subwoofer/s setup and location since every room is different. Multiple units usually reduce the rooms nodes and can give the illusion of faster response as it relates to their locations.   

Shortly after my Velodyne DD-18 I purchased two DD-12plus and for a short time I slaved the second 12 and the 18. The combination loaded the room leaving one node in an unused corner. The DD-18 was large and heavy and a bit of overkill so it was sold. A minor Room Optimization adjustment and the two 12s easily took up the slack of the missing 18. 

Velodyne DD Plus provides six frequency bands between 0 and 200Hz eight parameters of auto or manual adjustment and six customizable presets. Along with the test CD your main speakers in room performance is visible and any sub adjustments are visually rendered via laptop.  

If you have a wide range of placement options, go dual sub.

If you are locked in, go one big sub.

Room treatment always help.

Best,
E

+1 room treatments!