I bought a Randall cabinet a while back... it said it was rated for a maximum of 320 watts RMS.

1) What does RMS mean?

2) and even though a 150 watt tube head is much louder than my 220 watt solid state, can the cabinet still handle it? Could it handle a 320 watt tube head (if it existed)?
RMS = root mean square. its basically the maximum power you can continuously pump through the amp or whatever without melting anything. this is the power figure you should pay attention to and ignor the peak power figures. as for the second part of your question i have no idea
Quote by i_baked_cookies
Thanks Fetich.

Can anyone answer number 2?

watts are watts. A tube amp does not put out more power than a SS amp, It just sounds louder to our ears because tube amps produce more harmonics our ears are sensitive to, than a SS amp..