Zen circuit theory

Ok I give up. Who can tell me how a 10W output figure is calculated for the zen amp. With a Vds of 17V and Ids of 2A why doesn't the Pout = VI = 34W. Has this got something to do with the DC filtering Capacitors C3 & C4 or maybe resistor R2 for bleeding DC to ground. Can someone please clear this up for me.

Cheers

Dan
 
This may be it

Correct me if I'm wrong.

The voltage Vds = 17V is the Vpp of a sine wave output. To convert this to Vrms = 17/(2*sqrt(2)) = 6V. RMS power should be Prms = VrmsIds = 6*2 = 12Wrms.

If this method is correct why do I get 12Wrms output and not 10Wrms as described. If I am way out can someone please set me straight.

Cheers

Dan