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It is difficult to say below what frequency there is no sound registered, the lowest fundamental on a double bass is 41Hz and the A0 on a grand piano is 27Hz, both these fundamentals and at least one harmonic  thereof would probably be recognizable with a 25Hz second order filter.

The violinist Mari Kimura can with her instrument produce sub harmonics one octave lower than the fundamental of violin. Something similar might sometimes happen with other instruments to.

 The movements of the musicians is often heard or felt because the stage floor of a concert hall. The stage floor would have a fundamental frequency of maybe 5Hz, with the first 2-3 harmonics below 25Hz.

All this does not support the idea to call below 25Hz ULF based on the tone of reproduced acoustical  instruments.

Instead of defining the ULF range from the reproduced instruments, it is much smarter to do it based on the first resonance frequency of the listening room, because the frequency response of a closed room has a 12dB/oct rise in frequency response, due to the cavity effect starting approximately half a octave below the fundamental mode of the room.

This effect could be balanced by the natural 12dB/oct. fall in response of a closed box speaker, with a Qts of app. 0.7 and below its resonance frequency.

If the listening room is not leaky and with solid walls, a flat response down to a few Hz might be possible without equalizer.

Obviously one does not have to cross to a ULF channel below the fundamental frequency of the room to use this effect, but then the knee/ resonance frequency of the closed box should be placed there.

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