So you don't think I will potenitially damage either the speaker cab or the amp itself by having a lower speaker load than the R.M.S. rating of 150watts?
Your amp is rated at the same RMS output regardless of
which output impedance you select.
In a
tube amp the output transformer functions to match the impedance of the speaker load to the impedance of the output tubes.
It's not an exact science, a speaker's
actual impedance varies according to frequency, power, etc., but the impedance selector doesn't really have any effect on the amps output power; as in "selecting a lower impedance makes more power". There is a variance in output power with mismatched impedance (like a 4 ohm load on the 8 ohm OT tap), but it doesn't necessarily cause an
increase in power.
Generally a tube amp will make
maximum power with a matched load, that's the only
reason for multiple choices on most tube amps.
I wouldn't worry too much about running a 150 watt amp into a quad of 30 watt speakers, even wide open the amp will only put out 150 watts in short bursts, you should be able to hear the cabinet "begging for mercy" before the drivers give up.
