you overdrive the input stage of the amplifier which will cause the amplifier's input stage to clip and result in distortion. 6.5 volts is probably an RMS measurement though and the actual signal voltage may be a bit lower, so you may be alright with the gains at minimum.
you'd just need to use some sort of signal attenuator. I'm sure some company like PAC or FMOD makes them. Probably about $10-20 for them. Personally I'd just build one but that's not something everybodyc an do.