Here's a section of a Marantz brochure that explains the the tone controls -and the tone mode switch - for the 2275, courtesy of Ben Blish's www.classic-audio.com site (see #10):
For a good and simple visual understanding of the 20 - 20,000 spectrum have a look at this page. http://www.acesandeighths.com/studioeqtech.html
You can see exactly what instruments are being enhanced.