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
Don't think so.Call the manufacturer.
Maybe this site:
Agreed can calculate gain knowing voltage in and voltage out. The same holds true for phono stages. Gain (in dB) = 20 log (V1/V0).
I have volts in and watts out (into 8 ohms), so how do I convert watts into volts?
You should refer to the spec sheets. However, gain can be calculated but only with some assumptions.

The M300: if the 300W are into 8-ohms, and assuming the output voltage stays put, the output voltage is SQRT(300*8) or 49 Volts. If 0.9 volts is required to obtain 49 volts, then the gain is 20*log(49/0.9) = 34.7 db. Seems a bit high, but it's based on assupmtions.

Similarly the gain for the C150: 20*log[(sqrt(140*8)/0.6] = 35 db.
Thanks Gs5556. Based on your calculations, It looks like both amplifiers have basically the same gain, which is what I needed to know. Wonderful.