Hi Gwalt,
I always thought that the Titans combined the best attributes of horns and cones. From your description, I think I am right about this. When I was considering speaker systems, I thought hard about the Titans, but they were hard to find. I sent messages to Bruce Edgar (this was in about 2006), but I never received a reply. Had I heard from him, I might have purchased the Titans! What could have been... With your amplification, the seismic sub, and the Marchand amp/crossover, I think you have the best of the best!
Regarding my own integration of subwoofers:
I am not doing it "properly" in that I have the speaker outputs of the amps going through the speaker inputs of the plate amps and then directly to the binding posts of the Bens. This means that the Bens are running full range and the plate amps are receiving the signal from the amp and driving the sub enclosures. The plate amps are effectively low pass filters in this configuration and the drivers in the Bens are always running full range. No crossover this way, but it definitely makes the subs "bass augmenters," rather than actual subwoofers. It means that the 300B amp and the full range drivers in the Bens are running hard all the time. One of the benefits of doing it this way is that integration is a cinch. Just dial in the bass levels as you like. Change the Hz of the filter up and down and the gain up and down. No fussiness b/c the full rangers are running full range and there are no "hollows" to fill, just extra bottom. My tweeters are running in the same configuration, with just a high pass filter.