How to calculate gain

I have two pairs of amps, the first being ARC M300 Mk II's with an input sensitivity of 0.9 volts at 300 watts rated output and the others are ARC Classic 150's with an input sensitivity of 0.6 volts at 140 watts rated output. The second amps are basically a triode wired version of the first. How do I calculate the gain on these amps? Is there a formula out there in cyberspace? Thanks Guys