Hi Dave,
I spent some time thinking about the problem of bass below 50Hz, since that is about all you can expect rom a single driver.
I came up with the following options:
1. an active sub, like the Hsu, that takes output from the low powered tube amp used to drive the full range drivers, and then steps it down and amplifies it again.
2. A LARGE box (bass reflex) with very efficient drivers like electrovoice EVforce 15s ( i actually have a pair lying around) that will generate down to 25 HZ in a box of 20 cubic feet. These would be driven directly by the tube amp, in parallel with the full range drivers. SInce i don't have a LARGE studio at my disposal, and because of building costs for a large box, i think a powered sub would be the way to go, esp. if it is crossed at low frequencies. I am not sure if it will smear across the sound, if it is crossed low enough I do not think ti will. Also, i don't think the signal delay due to attenuation & reamplification will be at all audible to human ears.
I read a lot about the Hsu sub, and i don't see too many on sale used, so i thught they might be good.
The open baffle solution you use seems a high cost/size solution to me kind of like option 1. I am sure the results are great though. If you live anywhere near Tulsa, I would love to come by and hear the setup.
I am not sure about room modes not interacting just because the bass is directed in an Open baffle. I mean aren't the room modes because of the wave length? Does the directionality of the bass have anything to do with this?
One more thought: what kind of drivers do you use for the open baffle. Do they support a lot of travel? Most drivers would get damaged in open baffle, right?
thanx
-akhilesh