The run in (break in) is your ears not the speakers. The mids are crossed over to the tweeters at 2 KHz so they will not contribute to brightness.
If room treatments are not an option, does your avr have tone controls or EQ? you could try taking a little off the trebble to see if that sounds better to you.
My AVR does have room compensation but I'm trying to run as "analog" as possible. For my 2 channel music I'm bringing my flacs and some of them are 96k high resolution out via usb to my M-Audio Transit to keep them from being downsampled into my DACMagic and then analog out into a preamp with no tone contrlols to my amplifier.
