Sub? Can't Decide

"not disturbe the neighbors".. any sub IS going to disturbe the neighbors unless it is turned off.
If you live in a condo with adjacent walls, or an apartment, the part which is the killer for complaints are low frequencies.
If you want peace with neighbors, skip the sub. In a 10' by 10' room it is just asking for trouble. First off you will hardly get any bass you can hear IN the room, yet WILL create a LOT of bass travelling THROUGH the walls into the rest of the building. It will suck.
Just my two cents as a lifetime apt dweller. ((our opinions and mielage may and will vary.))

One reason i love my Magnepan 3.6 is the LACK of bass going through walls. The bass meeting around the edge of the dipole cancels out pretty good at the wall boundary.
Yet manages to sound good at the seating position.
I love them. And I do not bother the other tenants.

I use to own a Hsu and thought it was a very good sub for the money. Don't know anything about the other two.

There is a Linn Akurate 221 8" sub on A'gon right now for $995. The retail on this is $3750. I have one in my living room and it is a fantastic sub for both music and movies. It's perfect for a moderate size room -- not big enough for a big room. If you turn the gain low enough, it will fill out your low end in a very subtle way without bugging the neighbors. If you want to spend less, there are some open-box specials on small REL subs on vanns dot com for good prices.

Looking back, I sure wish my apartment-life neighbors had been half as considerate as Elizabeth is! That said, with your Mythos soundbar rolling off at 60 Hz, I think it's clear you should try a small sub.

Mateored is right. Get it dialed in with right placement and crossover freq, then set gain to just above audible. Most of the time it won't be apparent, but will add a base of compression that enhances everything else.

I have Hsu subs and they work with my system (and a great company to deal with), but I'm betting that you'll need something like the high-level crossover and Neutrik connectors, as well as other features found on the small REL's. For instance, you may end up raising sub's lower rolloff freq. due to your 10 x 10 room dimension. Strongly suggest you go for the REL 8" T2 from vanns, etc. per Mateored.

M&K sk 10 8'' 150 watt sub on ebay for 212.00 plus 28.00 shipping. I bought 2 in may and they are great for the money. I offered 190.00 and got the price even though I bought them a few days apart. You'll be surprised how well they work, and they are new.