It's not really major surgery to get to the woofer.
Tip the speaker on a side or on the back. Remove the 16 screws around the baseplate, remove the baseplate, and you can reach in and unbolt the woofer.

However, this is not likely to be productive in getting you the bass response you want.

Your biggest problem is trying to match the tone quality and sheer dynamic range of the Klipshorns. With efficiency of 104 dB at 1 watt input and a typical sub at about 91, you'd need forty times the power on a single subwoofer as you have on your mains (per channel) to match peak levels.

You have to decide how deep you want the bass response and how loud you want it to be at peak levels. THX spec is for 115 dB peak levels on the low frequency effects channel.

I don't know of a reasonable sub that could do it. I'm trying to fill a 4500 ft3 room and have decided that my LFM 1 Plus isn't quite at the level I want. A pair of LFM 1 EX ought to do the trick, though.

Steve