Subwoofer recommendation

Hi AudioGon community,

I'm looking for a used sub that would integrate well into my system and where I can purchase.
The sub is for music playback and I want it to blend seamlessly with the Revel speakers and not be difficult to place or set sub settings. My max budget is $400-500.

I have Revel M106 speakers and a Plinnius 1800 Integrated Amp.  The integrated amp has preout  and loudspeaker connections (no sub out).

Room is approx 12' x 12'.  Please also recommend cables. I've read a lot about subwireless systems, sub eq systems but its a bit overwhelming.

So far my considerations are:
Revel Concerta B120
Rel T1

  Thanx for your help AG folks, Fred 

" playback...blend seamlessly..."

The math is problematic IMO. After R&D costs, overhead and profit, not much to work with for materials and labor. Maybe better to go sub-less.  

I can't find information on the Plinius.
But, in any case, I would wholeheartedly recommend a Vandersteen 2w(q) sub(s), if you want seamless integration.
I also own a pair of HSU subs. They are quite good for the price, but the Vandy's really shine.

I ran Vandersteen subs for years. The Vandersteen sub system with the crossover before the amplifier does integrate very well. The problem is the crossover is not transparent. As I upgraded my system I moved to the battery biased m5-hp crossover which is more transparent than the stock cossover but still smears precise sounds like higher frequency plucked strings and well recorded vocals. For a while I ran the subs with rock and roll and would remove them for jazz. I finally got tired of this and moved the Vandersteen subs into the home theater systemEnter your text ...

12'x12' room.
Not be difficult to place or make settings.
Blend seamlessly.
$400-500 budget.
Overwhelmed by self equalization.
Buy used.

Self equalizing subwoofers, which are far less overwhelming than manual adjusting, would address five or six of your needs with the exception of your budget. 

A decent subwoofer will serve you well regardless of any future system changes. 
On the other hand some people find a poorly integrated subwoofer better than no sub at all. 
Velodyne's EQMax-8 might be worth looking into. Auto EQ and remote helps set up immensely. 
+1 jambalaya

Obviously I'm not keeping up with the folks in Morgan Hill.   (scroll down)

A long pair of Blue Jeans interconnects would wrap it up nicely.
I owned the Velodyne for over a year until we downsized to a smaller place.  Very good customer service too. Sent me a new grill quickly when one of the posts was damaged in shipping.  
Quality was very good. 
Capitola is about 45 minutes from Digital Drive. Using my third generation of Velodyne a pair of DD 12 Plus. I've been there a few times over the years because of my own mistakes and they have been generous. 

The EQ Max seems to be a well conceived yet economical DSP processor. At this price more two channel users will discover the usefulness of remote adjustability and get a taste of the DD potential. 
I am very pleased with the DSpeaker Antimode 8033 equalization. I would never again use a sub without this or something similar. It is far more important than the choice of the sub itself.
Your room is small, so you do not want a sub that goes very low or is too bulky. SVS SB1000 comes to mind, even if the combination with the Antimode exceeds your budget.