Simple answer is that Power equals the voltage squared divided by the resistance,P=(VxV)/R. At 6 ohms, you should have more available power than at 8 ohms, about 33% more.

The longer answer is that your amp may be current limited and you may not see any additional power.

The Rat. M80s, VP-150, QS8s, SVS PC 20-39+, OPPO, Onkyo 703s, Harmony 880 Sony 60" SXRD HDTV