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

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...

+1 to @jl35 

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 work...so 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?