Sub recommendation to augment Klipsch La Scala's ???

What’s your budget ? The new JL Audio are very good 
always buy a pr if possible for it balances out the room.
sysgy subs have apps and room correction for $2k for 2 
very musical and fast, Klipsch use cheap woofers ,that and their Xover ii not on par with their cost .i owned several .i rebuilt them
a total revelation once good Xover parts are used. Its s beyound me how they can chargeso much  ,Bob criterium May sell 15 inch 
upgrade drivers for them , for they are nothing special.
the Titanium mids, and I assume tweeter is Titanium ,thehereseys
i just sold had Titanium tweeter,and mids. Great potential 

Here is one for you the HSU VTF-3 MK5 HP SubwooferCheck out their reviews online I have had one for a year now and this thing goes down low and is very fast, you can't go wrong.

I agree with audioman58. Get a pair of subs.  

Must be musical, fast, and dynamic to keep up and give me the visceral kick drums I'm looking for. I listen mostly to Jaz / Blues / Classic Rock.

Are your two B-15s in the corners behind the La Scala's? Why not fill the other corners with the B&Ws slaved from the B-15s?

Two B-15s should be more than enough subwoofer. I don't buy into the speed and musical aspect. In my experience speed is usually associated with sub placement.

Along with location, poor optimization and EQ control hinders integration or musicality. Kick drum timing is often an issue with polarity which can vary from track to track on the recording recording itself.

Matching with a horn sub when your already tight on space? Not exactly sure how but Magico seems to do well with a sealed box with their Ultimate One's. 

While Revel's B-15 Optimization procedure is quite dated it offers much more control than many of the suggestions here. I wouldn't use Revel's suggestions of Y connectors and high pass filter. Are you using their LFO devise and disc or a 1/3 octave RTA?

Good luck with it.

If i can find another sub like mine I might just do that too...I have Heresy III and they sound great with my sub , but one sub can't really keep up as the volume increases... its tricky to find the right level for a single sub