@leroadrunnner Not crazy at all. In fact, I love the idea. The Ultimax 18 is really your best bet for something like this. Just keep in mind you are going to need a lot of DSP. So whatever amplifier you get you will need to dsp it. Chasing 10hz flat is going to be close to impossible. That is not saying you won’t get it, just probably not flat. There are two main reasons for this.
The first is every amplifier has some sort of high pass built in at low frequencies. The Dayton apa1200dsp is one of the better ones with a highpass as low as 16hz (3rd order). and I believe most of the inuke you can do a 4th order highpass at 17hz. Crown XTI cuts theirs off at 20hz (4th order). These are all implemented into the amplifier to protect the subwoofer (which is needed). So you will be relying heavily on room gain below 16-20hz. Which is normal. Second, you need that highpass to protect the subwoofer. This will make the lower frequencies harder to attain. Especially in IB setups, as they need the power. You will be dsping a lot of frequencies down to protect the driver. Just keep in mind most people show the graphs without the high pass implemented. Either way, to get low you will need to dsp it.
I think it is a great idea and I am really excited to see how it turns out. You could also do a large ported enclosure back there if you wanted to. But you could always start IB and switch later to ported if that is what you decided.
