This topic comes up from time to time (see below). I think it has more to do with the Fletcher-Munson curve (shown above) than anything else. What this curve shows is perceived loudness at various levels. At 70-80dB you need more bass for PERCIEVED loudness to be the same as you do at 100-110dB. This is because of the way your EARS work, not the speakers suspension or anything else in the audio chain. Some receiver/amps have loudness contour circuits built in. Press the button and it adds more bass and a smidge more treble when volume is low.

• 4 Pis at low volumes