Class A Tube integrated amp running 100% volume

All amps run at "full power" once you are beyond the volume control. This is fairly obvious if your power amp is separate from your preamp, but even integrated amps do this. It is just that the connection between the preamp and power amp sections is buried inside a single case.  So, there is no harm in turning the volume knob on your Theta all the way up and controlling the volume with your external passive pre as this is exactly the same thing you'd be doing if the Theta didn't have its own knob. 

Thank you for your answers.

Really great to know that there's help, even for simple questions.

if you don't like one volume control, you can use external that you like -- make sense. 

Often tube integrated amps don't have remote and placing it at volume maximum with passive remote volume control (such as monolithic) works also great in case if...

It sounds as if your amp's volume control is merely an input attenuator and not a gain stage as a true integrated would have.  Expected results would be different if it was a gain stage.  In that case running it at full might have an unwanted effect on SQ.  That you notice the opposite says it is just an attenuator, so running it at full removes it from the circuit, and no harm done.