Been asked before but... One sub or two?

Please - before someone tells me to read past threads, I have.  Still hoping to get some response to my specific situation, so thanks in advance.

My current listening room above my garage is approximately 15' D x19' W x 8' H with hardwood floors and drywall walls.  There is no room treatment other than upholstered furniture and an area rug, My speakers are approximately 9' apart along the long wall, 3' from the front wall to face of speakers, and 9' from my listening seat. Wall behind listening position is mostly windows (no curtains).  Speakers are Focal 1028BE (34hz-40khz~3db) on Gaia ll feet, with just a few degrees of toe in.. I also have a single Focal Chorus SW800V sub (32hz-160hz~3db) in the corner of the room along the same wall as the main speakers. Speakers are driven by a Marantz PM-11S3 (100w/200w @ 8ohms /4ohms).

 This sub was from an earlier system. I am trying to decide whether to upgrade and, if so, given the relatively small room size, whether one or two subs is the way to go.  My preference is a single sub that will complement the Focals but reach down closer to 20hz.  My budget is $1k-$2k.  Thoughts?
while typically 2 or 4 are recommended, 1 often works quite well. Really depends on room and system; careful placement and crossover and volume settings are crucial...I was thinking of getting a second REL and just today a friends’ son who is an acoustical engineer (or something like that), took a listen and said stick with my single sub...REL recommends 2...I have a friend with almost identical system, and his sounds better with 2 smaller REL subs...
You can always add a sub, but this is what I'd do to start:

Keep the existing sub.

Add GIK soffit traps in the corners. I like to stack 2 of them.

Add miniDSP to EQ the existing sub.  Most of what you'll need to do is kill exaggerated room modes.

A single Sub-Woofer will typically add to coherence with many mains.
The size of the drivers is more the focus than the room size.
A well driven 12"-15" driver will generally be a better choice.
A single Sub with two 8"-10" drivers will produce the same effect.
Low Frequencies are very long wave lengths.
Please note it is not necessary to use a lot of power for High SPL.
A Sub with a 'Long Throw' is very helpful to go low.
Also Stereo Subs may provide a better listening experience.
I just replaced a couple of older REL subs with two new REL's. Getting the blend to work in my small room is an ongoing challenge. IME, it is a ton more difficult to get two subs to work than just one.
Just when you think all is well, a bass heavy piece of music will play and the walls will join in or some other issue...a pita. Nonetheless, I do think once it all comes together, two are more able to minimize some room nodes. 
In the OP's room, since it is over a garage...this could mean even more difficulty in getting two to good advice above, try one first and see if that does it.
In before a post about a swarm setup....amazing. 

More subs are usually better than a single. However, setting them up and getting multiples dialed-in can be quite a challenge.  I run a pair in my main room and I spent quite a lot of time setting them up properly. I tried 3 and 4 in my system, but due to placement limitations, I gained nothing for running more than the pair, which is fine for me. 
 Thank you for your feedback. After much homework today, I have focused on the following conclusions ( but open to being challenged on these):

>Staying with one sub (for now) mainly due to relatively modest room size and potential integration issues;

>Buying new rather than used.  All of my equipment is "gently" used and I have had zero issues with any items ( almost all purchased through Audiogon). But too many problem stories online regarding used subwoofers;

> Since I am buying new, I am focusing on the $800 SVS SB 2000  PRO, an new addition / upgrade to the SVS line.  As such, if it proves to be a good fit, I will be able to match it with a second one if I end up taking that path.  The SB2000 has been around for a while, is a sealed sub which reviews claim provides a tighter response.  The newest version adds some additional wattage, additional range (19hz-240hz~3db) and control via and app used in their higher end subs.   Considered the SB3000, but seems like overkill for my needs and costs $300 more.

> While SVS seems like a home theater brand, I have not found many people bashing it for two channel music.

Anyone have direct experience with these?  Any thought on sealed vs ported for music?