Some of my preamps (I have many, mostly vintage) have a loudness button. I lean a little to the purist side and I do not use it because I can listen at higher levels. At low levels a well-designed loudness function does improve the frequency balance, imo. Some are better designed than others. For instance H-K designed a "Phase-Correct Loudness" (as they call it) feature for their Citation 21 preamp. It boosts only the very low end and can be used to extend the low frequency response of smaller speakers, as they say in the manual. They go on to say that high end boost is not necessary, from their listening tests. A look at the schematic reveals much more than just an RC network. It is an active circuit with four transistors per channel in addition. To my ears it is not unnatural sounding.
