I started this project a long time ago with a skilled woodworker friend. It's a 23" inch wide (to match my combo amp's width) with a 3012LF (we changed from a BP102 once this came out). We got to sealing the second side and then he was unable to set aside blocks of time to have me over, so he worked on it little by little until two nights ago when he had me come over for the final touches (electronics and access panel) and then today I picked it up painted with casters on.
We tested it in his basement, pre-finished, and I thought something was wrong. I could hear highs even though there was a passive crossover. I hooked it up to his home theatre and didn't hear any thump. I was a little disappointed. We A/B'ed it with the subwoofer from his surround sound, and I noticed a real difference. The T39 was almost invisible -- the bass from the radio station we were listening to came from nowhere. No honk, just clear low end. I don't think I've heard clear, harmonic-free low-end before. We plugged in the theatre sub, and yep there it was, the obvious source of lows (and higher up thumps). I changed back and walked around, and the bass was more apparent the farther away I got (around corners, away from the highs) as I walked back I could hear that it was still there, just not distorting the box and making itself obvious. I understand now how people can buy the myth about horns being louder farther away from the speaker. The bass is everywhere, but there's no kick/thump/honk made by the box itself.
Today I went back with more equipment to test the crossover and my bass and amp to give it a test run. Sure enough, the crossover was working, but it sure isn't a brick wall. The frequencies above 120hz keep coming, helped by the natural ease that speakers can make them loud. At about 500hz, it's much quieter, but I can still hear 2khz sine waves. I put my combo amp on top and played. After all the sine waves, hearing a natural sound made it sound great. All the vents and the floor above were shaking. We took it outside and I played on the deck. It was like outdoor concert level (though, I was standing right next to it.) The amp head was buzzing pretty bad, making it hard to hear the lows. (I could still feel them!)
Since I've got it home, I tried it outside again, having fixed the rattling head and with more time to pay attention to tone. It's fantastic! The sub seems to add none of it's own character. It still sounds like my amp, but without the farting or distortion I had when I pushed it too hard. Playing my b-string is great! It doesn't quiet down or change the harmonic/fundamental proportion, it just keeps going lower at the same volume. I have to EQ down the lows to compensate for the difference between the combo's speaker and the T39.
I've got a couple outdoor gigs coming up where I'll be using this (usually my combo is enough) and I'll review it more then. I plan to put duratex on it when I finish my DR200s, but for now it's been primed and painted with a mix of rustoleum flat black and gloss black. The mouth braces were left off by mistake, but we'll add them when it's time for duratex.
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with a some growling. I tried to see if I could get a clean sound by using my solid-state channel (instead of the tube preamp), but the power amp seems to have been adding a touch of distortion in the mids. I wondered if it was a problem with the T39, but it doesn't seem to exist when I play music through it. Maybe it's from pushing too hard, or running above 8 ohms, but it sounds good, I can't complain. Hopefully it's not something damaging. 
My amp and the guitar heads were on one long extension cord in one door of a school building, and the power amp for the mains, along with the power amp for the monitors, were on another extension cord running into another wing of the building. The amp to the mains would cut out, wait a couple seconds and return, but the monitors stayed on. My amp cut until restarted (and cooled), but the drums, and guitar heads stayed on. The power amp was in the sun, and both were under a canopy that, while providing shade, somehow turned the stage into a sauna.