Headroom question about Carvin BX1500
I was at the Carvin store today, A/Bing the BX500 and BX1500 through a 4ohm BRX10.4. Both were room shakers, but I noticed that when I played a low C# on my 5string, the BX500 distorted and the BX1500 stayed clean. Since I was only playing out of one of the BX1500's channels and getting 450w at 4ohm, how was there any difference in headroom? It's not like I was running it bridged or something...is it just because each side of the head is capable of 750w so there were 300w to spare even though I was only pushing 4ohm?