How loud and "low" must a sound be in order for one to feel some physical sensation in response? I've read that below certain frequencies humans can be expected to "feel" sounds in their chests, and I assume amplitude plays into this as well, but at what point, specifically, does this normally occur?
Having played electric bass guitar for 48 years, I can offer some rule-of-thumb guidance here.
You can begin to "feel" the sound from a bass guitar amplifier at about the 100 watt RMS level, assuming good coupling between the speakers (2 x JBL D140F) and the air, if you are about one foot from the speaker cones, and playing the lowest note (fundamental E). Farther away than that, the bass will still be deafeningly loud (to normal ears) but you won't feel the bass.
At the 1000 watt RMS level and assuming 18" drivers and a detuned, extra low string (fundamental B) you can feel the bass notes several feet away.
The human body is supposed to have subsonic resonances in the ~10Hz range but to excite them at any significant range (say, tens of feet) requires a lot more than 1000 watts of radiated power.
Note than since 746 watts = 1 horsepower, we're talking very, very loud!