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

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!

1 good sub is enough unless you have a ridiculous sized room. Most problems come from placement and room gain. Two subs can fight this but at added costs. Just move a single good sub to a best place and treat the room. The only true accurate way to go. And ported will get you lower, and no advantage over sealed. All depends on the design and quality. Room correction has come a long way and it will only get better. Remember absorption for room correction can be frequency selective, acting like an equalizer and changing the original sound. Diffusion can scatter the reflections without taking away or adding to resonances in the room, making certain frequencies louder or softer. So you have to research the room you are in and the decay rate RT60 and use a computer and a microphone and nail it in. Get the first curve and fine tune it to more accurate. Plenty of free software to do room correction. Good luck

For HT, ported is fine. I hate them for music.