Frankly any one of the SVS PB13Ultra, JL F113 or the Velodyne DD18 are going to be excellent in either mono or stereo (it doesn't matter much but you may get a more even in room sound if you have two subs to play with and you have the liberty of placing them in the best spot). I'd guess that the DD18 will beat the smaller drivers in terms of absolute SPL.
The advantage of the PBUltra13 is you can play with the ports (all three blocked, or any or all three open) which will change the "Q" or resonance of the sub - this may help in integration with the bass of your speakers (I am not sure what sort of response the Klipsch La Scalas will have but they look like bass bins or folded horn designs so a resonant sub with higher Q will integrate best. If you can find group delay measurements on the Klispch or an anechoic frequency resposne then it will be easier to judge what would work optimally. Suffice to say that getting a very tight overdamped musical sub may cost you a lot more money than you need to spend and also give you lower SPL then something with greater resonance or group delay)
The advantage of the PBUltra13 is you can play with the ports (all three blocked, or any or all three open) which will change the "Q" or resonance of the sub - this may help in integration with the bass of your speakers (I am not sure what sort of response the Klipsch La Scalas will have but they look like bass bins or folded horn designs so a resonant sub with higher Q will integrate best. If you can find group delay measurements on the Klispch or an anechoic frequency resposne then it will be easier to judge what would work optimally. Suffice to say that getting a very tight overdamped musical sub may cost you a lot more money than you need to spend and also give you lower SPL then something with greater resonance or group delay)