Subwoofer

Ricred1, I hope you don’t mind me chiming in as I have enjoyed this thread and am considering adding a sub to my system.

I am driving a pair of Esoteric MG 10s in a room ~12.5 W x 15 D x 8.5 H. Though I am convinced that more than a single sub is best I would like to start with a single. I also think I would like to continue to use my 10s full range.  They drop off rapidly below ~50hz. I would be willing to buy a used sub to experiment with as I could resell it if desired. I believe a close box might be best, with variable phase. Any suggestions on a sub or my thinking?

I like Velodyne's DD series because of the room tuning and ability to play below 20hz.

Mesch,
I've owned Martin Logan Depth i, HSU, SVS Ultra and now the JL Audio F110s V2s. I have dual F110s with the CR-1
Crossover. I prefer the F110s over my previous subs. That being said, I'm selling my subs, crossover, and Aerial 7Ts to purchase Monitor Audio PL500. 

In a room your size Mesch, I think the DD Plus 12" would be good. Velodyne has a chart on their site. Adding another later will work great.

First off I am a dealer for Velodyne, so there’s that. I have a pair of the DD10+ (on sustrum stands about 20" off the floor which I felt helped articulation,) complementing Tad CR1’s.  I run the TAD’s full range and the subs of the amps, which I think is preferable (at least with my BHK 300’S)
One of things no one seems to mention is that bass levels in mixing are rarely consistent.
Add that to the loudness curve based on playback levels.
So an added benefit it using the subs as a tone control. (of course an expensive one, and a feature most of us don’t have in our preamps)
I have the remote buttons on the velodyne set up with 5 different slopes so it’s easy to fill in recordings with weak bass, or for late night listening. Even sub optimal live concert streaming like grateful Dead ex member shows from the Capital theater in Port chester NY or live jazz from Smalls and mezzrow here in nyc are enhanced with a sub. It’s kinda fun to expand the bass from these Web streams and hear the 7th Avenue subway go by, just as when I’m actually in Smalls jazz venue.
On proper studio recordings, When I mute the subs and listen, I feel like it sounds a bit like truly excellent hi fi. Adding the subs back In - even though they are coming in only below 40hz, sounds like musicians in the room. It subtlely fills out voice and other instruments higher up in a way that seems counterintuitive.
I guess I’m a tweeker, but I really enjoy taking a few seconds per track or per album, and optimizing the bass level and sometimes slope to get the most realistic feel for the moment. This is probably in stark contrast to most, who get a reference or flat response and wouldn’t dream of altering it to taste.
I have never compared other subs in my space so I can’t comment - but of course I’d love to hear JL’s latest just for comparison.