Your room is certainly large - close to three times the volume of my den. That's going to be a challenge for any sub to really fill with low frequency sound. I have a few thoughts, but I'm going to approach them somewhat "backwards" from your original questions.

Could the LaScala's be modified to serve as "subwoofers"? Based on these specs , I doubt that they'd serve the job particularly well. The LaScala's have a -4dB point of 53Hz. As a comparison, Outlaw's smallest sub has a -2dB point of 28Hz (the -4dB point is probably close to 20Hz, or more than an octave below the LaScala). Your existing mains (the Klipschorns) actually go 20Hz lower , making the LaScala's pretty superfluous. I doubt that modifying it would gain you much - and certainly not enough to provide the Klipschorns any appreciable help.

You would need to look at the power requirements of any subwoofer that you would be driving from the Model 750. Also keep in mind that the 750's power output will increase with a lower-impedance speaker - and many passive subs are going to be a 4 ohm load. That being said, there are also very few commercially available subs that are actually passive subs. The subwoofer market is dominated by powered subs that rely on their own built-in amps. Also, I would not bridge the channels of a 750 because the amp wasn't designed for that.