[fhchl]

This is a common psycoacoustic effect and probably not due to the loudspeakers or the room acoustics (which are linear): the (perceived) loudness of a tone is frequency and level dependent [1]. This makes sounds more bassy at large sound pressure levels.

[1]: https://en.wikipedia.org/wiki/Equal-loudness_contour

[ttpphd]

Loudness is also distance dependent, as in, not just how far away the sound source literally is, but how far away you think it is.

[newaccount74]

Some hifi systems have a "loudness" setting that raises bass+treble to compensate for this effect at low volume.