Good post. But regarding the quoted portion;
1) The beam should be fine from a manufacturing stand point as it would be to standards, but they do seem to put them in untreated, so perhaps corrosion over time could reduce its performance? But I don't think that is worth considering.
2) "Poor construction and insufficient precision", well that is a guarantee from cheap home builders! I think these steel beams just bear on some timber that is bearing onto the top of the wall. There might be a few fixings to keep it roughly in place, but there certainly isn't a proper bolted moment/shear-plate connection or anything. But I doubt this "sloppiness" is a problem either (as it is fit for purpose in supporting the roof against the elements)
3) "Too high of a load", is not a problem here because modern roofs are designed to engineering standards that cover the extremes, so the roof should be fine (and it's not worth considering increasing extremes for this discussion).
4) "too loud of a stereo on top of all of it, would the stereo be the one to blame?" - yes, lol. The roof is designed to handle the various weather related loads, but they don't consider a potential "weakening of performance" due to resonant frequencies induced by a subwoofer.
If jsilvela's post is correct, 29Hz is an easily producible frequency. This post was sort of tongue in cheek, not a true concern of mine, but at the same time there is a nugget of truth to it. I presumed the frequency might be single digit too, but apparently not. It might actually be a benefit that roofs like mine are made from timber and without tight tolerances, because that way they maintain a sort of flexibility. Some houses are made from light gauge steel though.
But anyway, if this were a real problem, I'm sure someone's roof would have fallen down at least once before!
Sound pressure itself, as some of us pointed in this thread, is hardly going to be a problem. Resonances may be, but, as most of us here care about sound quality, resonances by definition we like to minimize.
When I built my DIY system, in room I had a problem with floor resonance. It was literally vibrating under my feet. It had nothing to do with SQ, but at higher SPL, the surrounding stuff which were coupled to the hardwood floor (kitchen utensils included), had started singing by itself and were subjectively annoying. During transients, stuff like beforementioned, including windows and doors were louder than the program material. To me the solution was acoustic suspension, basically elastic feet to suspend tower speakers and the sub.
I can confirm that this did absolutely nothing to further improve sound quality, but the floor born resonances are very much attenuated and no longer a problem. Interestingly enough, stuff which are on the vertical plane (walls), do still vibrate on high SPL itself which is no surprise, but they don't buzz anymore. From that experience I can't help but come to a conclusion that resonances are more complex than we may think, possibly need force on both vertical and horizontal plane and leading to a some kind of interference which may even pulsate at an entirely different, lower frequency. Like in listening tests, when we play two frequencies which are close enough together and suddenly hear a pulsation at the differential frequency. In that sense, I don't know, but maybe it can be possible that the surface ratio of in room horizontal and vertical planes not only creates the room modes of sound pressure, but also resonances containing frequencies we can't audibly perceive because they are masked by the actual music.
I remember back in the day when I had one of those cheap downfiring subs. It was producing a dreadful one note bass through the ports and when cranked, at lower frequencies it started jumping up and down on it's plastic spikes. The resulting resonances were such that, below port tuning and high excursion it produced barely any SPL, but in turn was shaking the floor vigorously. Once I got out on the balcony when a bass heavy song was playing and the experience was scary to say the least. I thought it was going to collapse under my feet. Temporary solution was to flip the damn thing on it's side, suspended on some books, because that's what most of the students have. Long term solution was to get rid of it.
So, as I said before, you need a lot more moving mass, and a complex resonance, like throwing a brick in a washing machine, or, if you like it to be a bit more musical, an orchestra of cheap downfiring subs, for a multitone resonance, pointed the right way. This could possibly bring your entire house down.
To further illustrate, I can't help but thinking of Tesla's earthquake machine. For those that aren't familiar with this concept, it is easy to google it. But here's a working model:
Note the multitude of resonant frequencies.
EDIT: To me it's funny that subs I described sound quite similar to this machine. They have a very high port velocity and have more than 100% THD, so for one frequency input, they play many more. And they shake surrounding stuff like this.
Sorry about the long post, but I must conclude that if you care about sound quality, and are moderate in SPL, there's nothing to worry about.