Daniel, Koss' K-6 'phones were considered the poor man's Koss Pro 4AA . In
Allied Radio Shack's 1971 wish book, were priced @ $26, with a 10-15,000
hz(alnico derived?) response curve. Radio Shack also sold a variant, badged
"Custom Pro", with "exclusive bassport feature (which) employs acoustic
principles in design to extend bass & expand highs for full 20-20,000 hz
response". Price was $22.95 . You can see Michael McDonald wearing these
'phones on the back of Steely Dan's "Katy Lied" album.