Subwoofer Recommendations

I am looking for a single subwoofer that will compliment my JBL L100 Classic speakers.  This is my office system and I only want 1 subwoofer (no swarm) to add a little oomph below 50hz.  I will be using it with my McIntosh MA6600 on the second set of variable line outs.  The sub has to accept line in, have a phase control, level control  and adjustable crossover so I can cross it over under 80hz or so.  I will be still running my JBL speakers full range.  Price is up to $1500.00 new or used although keeping it at around a grand would be great.  This is the only way I will hook up the sub and I would like something that does go deep.  I know there are probably hundreds of subwoofer threads but I have never looked at them.  This is just something I came up with today so any help would be appreciated.