Why does my 35 watt tube amp draw 300 watts of power

At the other end of the power consumption spectrum I have 500 w/ch Class D Bel CAnto ref1000m amps that draw 5 watts/ch idle.

Class D technology is a game changer.    If not for having discovered success trying Class D,   I might well be down a totally different path than I am these days.

The way I look at it these days, Class A (many tube or some SS amps, very inefficient) or Class D are the two pure approaches to consider at each end of the power consumption spectrum.   Then there is Class A/B which most SS amps are, that prior to Class D was teh technology used to improve power consumption and retain "quality sound"   Some will argue Class A/B is a compromise approach  ( I might tend to agree) that might still yield very good results but not as easily or as often as Class A.

IMHO, in general, these days, I would rather go with either Class A or Class D amps for pure sound quality.  Either done well and matched to the right gear can deliver top notch performance, subject still to personal tastes as always. 

Class D is fairly mature these days and offers a nice bundle of value and performance in a small package that will appeal to many. 

Class A amps, will tend to be larger and cost more to operate but some may still see the value if its "that sound" they like and cannot find elsewhere.

Tube amps are only 5% efficient.