The "loudness" button is a type of bass boost circuit that compensates for the Fletcher-Munson Curve; that is to say that at low listening volumes, the ear doesn't percieve bass as well. So the loudness control boosts the bass so it sounds full and punchy when you have the volume turned down.
Don't play the system loudly with the loudness button engaged! Your amp only has so much power, and you stress your system when you boost + play the stereo loudly.
WerTicus, without dredging up the old debate about the audibility of sounds over 20k, I'd suggest that if he's got an inexpensive 10 year old amp, his speakers likely aren't new and spendy, either. I'd wager a 10 year old amp is much more likely to reproduce sounds over the accepted range of human hearing than his Sony speakers are.
SACDs don't
really have much of any content over 40k, and certainly none at 100k. That extra bandwidth is used by DSD's noise shaping alogrithym to move noise out of bandwidth. No transducer could hope to play a full scale sound at 100k, anyway!
Still, any excuse to buy new gear!
It would be a good idea to buy a new amp, as technologies have changed a lot over the last 10 years. Where do you live? SACD isn't really all that expensive, with universals starting at $150 and an actual Sony or Philips machine starting under $200.
