You need to find the full specs of the amp. In most every case, amps are designed to take speakers specified as low as 4 ohms, so you should be fine with 6. There normally is a sticker or printing on the back panel stating the minimum impedance. I realize this is an older amp and may not have that.
If it is a well designed amp you should yield more output with the lower impedance.
6 Ohm speakers are very common (these days).
Samsung PN51F8500, Emotiva UMC200, Emotiva XPA-5, Samsung BD-H5900 BluRay, Dish Hopper, Furman Elite pf-15 conditioner, Dali Ikon (Ikon6, Vokal2, OnWalls) Outlaw Audio LFM-EX subwoofer.