The SVS PB-17 Ultra advertises a range of 12-220Hz at -3dB. I imagine it could play a pure 7Hz tone if you turn it up.

And most speakers can play infrasound for many non-sinusoidal waveforms [0]. They'll drop the fundamental and some lower-end harmonics but can still give a sense of what it sounds like

[0] https://szynalski.com/tone#7,saw,v0.5

Who said the source has to stationary? Doppler shift for the win.

If I feed a 7hz input to some cheap hand-made thing like https://www.youtube.com/watch?v=liSEwqdq7aA , will it not vibrate at 7hz and thus produce a 7hz "tone" (disregarding that humans won't perceive that as sound, at least not the fundamental)?

You can with your hands, just shake them