I have two 1*12 8 ohm cabs rated 250 RMS. Now i'm looking for an 500 watt amp. In an ideal situation this amp should give 250 watts at 8 ohm (with one cabinet) and 500 watt at 4 ohm (with two cabs connected). However, most 500watt amps give 300w or more at 4 ohm. My question is : How harmful is a 300 watt or more amp in a 250w RMS cabinet? is it enough to be cautious with the volume knob? Or are there other more fundamental problems?