What B said. The human ear can't hear it, so why bother producing it. Anything below (I think) the G on the E string and most humans are hearing the second harmonic. (If I got that wrong, sorry. It's late and I just did 4 hours of homework. but it's somewhere in that neighborhood if I remember correctly.) Which is fine. Our brains are pretty good at filling in the blanks.
Short version. It can be done, but why bother? It would be like putting a dog whistle in line with your rig to produce all the harmonics on the other end of the scale. A lot of effort for no return.
Actually, the Low B note is clearly audible. Anybody with a decent speaker and a sine wave generator can prove it in two seconds.
How the mythology that 31 Hz can't be heard has gotten traction on Talkbass is beyond me. If there were ever anything easier to disprove, I don't know what it would be.
What's even more bizarre to me is why it even matters. If I understand the narrative, some people believe that since these low notes can't be heard (ridiculous...), but only felt, in a tactile way, that they needn't be reproduced at all. That's even sillier than the contention that they can't be heard. Why?
Simple. The tactile experience is part of the musical experience, that's why. Do you want your audience to not feel the bottom of your band? Do you want to limit the low end on the bass drum, the same way you want to limit it on the bass guitar? How about the pipe organ at church, which goes all the way down to 20 Hz or so. Do you think they build those massive pipes for fun, or do they intend that the audience "feel" that low-frequency energy on their chest or gut.
The third erroneous narrative which seems to have taken hold is that, since the fundamental is, in many cases, relatively quiet compared to the 2nd harmonic, that the fundamental, for some reason, need not be reproduced at all. Why? That makes no sense at all.
If anybody has any meaningful explanations for the tenacity of these silly myths, I'd love to hear them. But I've read enough of these threads to realize that these notions are accepted as a matter of faith by some people, and the facts don't enter into it.
Allow me to summarize:
1) A frequency of 30 Hz is clearly audible. This is not a matter of opinion, it's a fact.
2) Very low frequencies are part of the musical content of your performance, and even if they weren't clearly audible, which they are, that would be no reason to deny your audience the tactile experience of feeling the low bass in the room.
3) The fact that the fundamental can be attenuated with respect to the 2nd harmonic is irrelevant to this discussion. A decent loudspeaker should reproduce the various frequencies in their proper proportions, regardless of the characteristics of those proportions.
Having said that, I might directly address the concerns of the OP, who said, "I have not found a SINGLE cabinet that goes below 28Hz at -10dB. I want 27.5Hz FLAT!!! I know it can be done. Does anyone know of a cab that does this already, or do I have to make one?"
The answer to your question is that you are very unlikely to find a commercially produced cab with that capability. It could easily be made, but would have to be very large, or very inefficient. It will be a tough nut to crack. I would recommend that you look for a model which uses a sealed enclosure, such that you could then EQ whatever low-end into the thing you want. I'm not particularly knowledgeable on Bag-End equipment, but my loose understanding is that they make some cabs sort of like that, and perhaps suitable for someone in your situation.
Sorry to not be more helpful, but at least I have an opportunity to address some of the nonsense people spread on these boards.
Thanks,
Andy