Question on amp power and speaker ohms.

I have a 30 watt(15wpc) tube amp. I can set it for either 4 or 8 ohm speakers. I seem to recall seeing ss amps showing different power ratings at 4 and 8 ohms. So if I were using 4 ohm speakers, would the power output be greater than 15 wpc?

Also I currently use 6 ohm speakers and have the amp set for 8 ohms. Is this the right way to go?
Try it set on 4 ohm for a while, then on 8, and see which you prefer. Most tubed amps output less power into 4 ohms than 8(an output transformer winding thing). My Maggies are 6 ohm nominal, and I'm running them on the four ohm taps of my amps.
Most tube amps don't double power output with halving of impedance. 15wpc is all you get.

Try both taps for 6 ohms. Use which ever sounds best.