Cat,
Response up there (15 kHz to 20 kHz) simply doesn't matter. There is no musical information of any consequence for any musical instrument in that region ; it won't affect or change your judgement of the "sound" of a speaker, and you'd have to be a child or teen-ager to even hear frequencies that high.
"It's all in the midrange" is a bit of an exaggeration, but our judgment of a speaker's sound quality is largely confined to the spectral balance of the mids and upper mids (to about 8 kHz) and the bass extension and balance relative to the midrange.
Regards,
Alan
