Well, the DT25 is actually 25 watts when running Class AB Pentode mode...theoretically...Bias is generally 70-80% of that...So call it right around 20 watts...If you change to Class AB triode mode, then the power drops a bit...Class A pentode is right around 15 watts and triode mode is a little less....

Speakers have two ratings...RMS and peak...99% of the time the peak rating double the RMS rating...As long as the signal is not clipping (DC current), any speaker can be run well past the RMS rating. The nice thing about a tube amplifier is that they basically can't put out DC current...It's the nature of the beast...When a tube maxes out, it compresses and you still get a sine wav...

You would need to be a little careful with a 50 watt head...The amount of power that any amplifer puts out is proportional to the master volume (input signal)...Comfortable home listening is actually on a couple of watts of power...two cabs in parallel would be better with a 50 watter...but no problem for low to moderate volume...

If it makes you feel any better, professional PA systems that are properly powered actually use amplifers that are 2-4 times the RMS rated power of the speakers they run...In that application it is all about clean headroom...amps never clip and the speakers never see compromised power...It is really amazing how much power a speaker take past it's rating when the signal is clean...