I've tested with the 807 single CW Xmtr into a MagicBox(MB).
It's power out is 12W(max) at Vp=350V into a dummy load.
I put the Xmtr into my MB and saw its power out into a dummy load.
The power out is about 8W. The 4W is loss!
Then I put another QRP TX of 4W into MB I got 4W no loss!
Maybe the power loss depends on the input power of the MB.