Tube vs. Solid State-Basic Question

So here I am with my first all tube amp, the Manley Stingray II. While I wait for my Merlin TSM-XMr's to be delivered I've got the Stingray connected to a pair of Definitive Technology BP7004 towers in my theater room. I also ran them with a few other amps....testing them with a Cambridge 550A amp with 60 watts. I was able to play the speakers with the Cambridge well enough, but on tough passages at higher than medium volume the sound got congested and even distorted above that level.

So now I connect the new Stingray, switch to Triode-Mode (just 18 watts) and I get equal volume (or more) with beautiful sound, dynamics and imaging....and that's 18 watts in triode mode! The extra watts of the Cambridge did not appear to be of any use. What's up with that?

So....can some folks more educated on tubes explain to me....like I'm a two year old how the low powered Stingray plays so well with so little power?

Next up was to switch out of the triode mode and get the full 35-40 watts from the Manley which could drive the speakers very loud in my fairly large theater room. While I'm VERY happy with the Manley Stingray, I'm also a bit confused at the power the thing seems to have in spite of it's lower power. In fact my fat Denon receiver (110 watts) cannot keep up with the Stingray.

All comments welcome!

Bob