You might consider something in the Dynaudio line. Their small speakers are noted for their great sounding bass output, which belies their size, and their power-handling abilities.
The Dynaudio Excite X12
is only 11.2" tall and has been garnering rave reviews from anyone who reviews them. They are ported to the rear, so I don't know if that works for your speaker placement. The reviews universally praise this speaker's bass extension and dynamic range relative to size.
If rear porting is a problem, Mordaunt-Short
makes a front-ported small speaker. It's 12.6" high though.
Then there's the PSB Image B5
that comes in right at 12.5" high with a sensitivity of 87dB and power handling of 150w. These are extraordinarily linear and are known for a lifelike transparent midrange, especially for $449/pair.