[Amps] Maximum tolerable SWR for dual 3-500Z amps

[jsb at digistar.com]

Is there a rule of thumb for maximum permissible SWR for 3-500Z amps at
given levels of output power?

Say, 300 watts for 2:1 SWR or 800 watts for 1.5:1 SWR, etc?  For SSB and
CW modes?