I'm not familiar enough with the design to say too much. I will say this much. A bass horn needs to be big and anything below about 100 Hz is usually going to be compromised. When you shrink a bass horn, you can expect a peaky response and ringing. In a sub horn that is a given, so you have to cut down the bandwidth and make sure you aren't using it where it is ringing. That's why mine only goes up to about 40 Hz or so. You don't have to compromise as much with a 40 Hz horn, but one question is how how do you want to run it? Bill's designs are generally real world pro boxes, all of them seem to be compromised out of necessity.
In a very solid room like you have now, the room itself is the weak link in the chain, it will tend to swamp things like this. But if you get it really well sorted, and get bass decay completely under control, then issues like this are more likely to stand out. TD18 in a sealed box with a bit of power behind it, EQ and bass traps - that has the potential to completely redefine fast bass, and it's about as good as it gets. The driver has 28mm excursion from peak to peak, in a room like yours full of gain that is a lot.
You mentioned amp power. The thing I've noticed with driving bass is that underpowering it doesn't make things boomy as a rule, you just lose dynamics and authority. This became clear to me one day when comparing a 250w plate amp such as we find in many subs, to a meaty amp - a Crown K1 which is a fairly expensive pro amp. We ran them at what sounded like about the same level, but the difference was jaw dropping. We could not believe it. The extra grip, control and authority was staggering. We played a track with Eric Clapton live in Hyde Park - layla where the bass guitar was thick and punchy. The little plate amp should have had enough grunt to play at a level that wasn't crazy, but the Crown totally nailed it. My suggestion - put a good pro amp behind bass drivers. You'll save money and they do bass better.
The question you have to ask about the bass horn, is what do you want to achieve? Do you just want to have fun building the thing? In that case, go for it! I have a feeling you want to do it "just because" ... for the love of horns.
Do you want it because you love the idea of valves driving the bass? IMO that's the wrong reason to do it. To do bass well you need things that valves aren't well suited for. IMO, the real advantages of valves live everywhere else.
I'd want to see a real measurement of the horn in question, an unsmoothed warts and all one. Last time I saw BFM measurements they looked a bit prettified, a bit too santised.
One more comment. With bass horns, the way you load it into the room makes a huge impact on the size and degree of compomise. The best way to do it is with the mouth firing into the corner so that the corner becomes an expansion of the horn. Have you seen Avantgarde bass horns? Silly question! Ok, they put them in the centre of the room because that is where they fit. But they could effectively double up, by splitting them in two and putting them in corners. Each horn is a module, which is like a quarter slice of pie. Now stack those modules in the corners and the quarter of a circle horn in one corner now equals the half of a circle version that was sitting in the middle of the room. I did some modelling of those horns when I was tempted to make something similar. They work out to be a good design for 40 Hz and you could actually DIY one of those with some sheet metal for the curves. IIRC the radius of the full circle is about 1m, with the height of each module about 700mm or so. Stack about 3 in each corner and you have a pretty serious bass horn. The AG version has so many bass drivers that you can then EQ the bottom octave in, and easily get to 20 Hz. It's quite a clever idea. I would have done it, but I realised that it was going to be a bit big. My bass horn is under the floor.
When I modelled the AG bass horn, I quickly realised that their design when placed into a corner, it's not a cut down compromised design. You could build it with sheet metal that is thin enough to easily bend, fitted into a slot in the top and bottom, laminated together with a stack of contact adhesive, or construction adhesive. It has some challenges, but it could be a fun project.
So what happens if you take those horns and move them away from the corners in the middle of the front wall? You have to make the horns now TWICE AS BIG! Bring them out into the room, so that the wall behind is no longer reinforcing, so the mouth is out into the room. Now you have to double the entire thing again! This is due to loading. So if you have your heart set on doing a bass horn to get down to 40 Hz, and you want to do it right, the smart thing is to load it into the corner so the corner acts as an expansion of the horn. Ideally do it with modules so that now you've created a mouth that runs floor to ceiling.
Only problem is, now they are fighting for bass trap real estate. Dilemmas, dilemmas ...