OP the 24 hour break in is actually a burn in. One of the few times "Burn in" actually applies. The RIGHT way to "CHECK" new valves is to BURN them in for 24 hours. So when you see the "Burn In 24 hours" they actually are or they were suppose to have been.
They set up grids and let them percolate for 24 hours. Normally keeping an eye on drift and then they write down the numbers and match for sets.
It was for failure that is the only reason for "BURN IN". If there is no failure then they go in a a unit and "BRAKE IN" the rest of the way. The truth of it is usually by 24 hours or so, how that valve is going to act and sound is pretty close. They don't change sonically very much after that..
SO from a "seeing if they are going to fail" perspective it is important to leave them on for 24 hours STRAIGHT..
From a break in perspective it is just about TIME, your doing two different things...
I say plug and play... AND KEEP and eye out for the first couple of hours.. Vary the volume a bit. Shut down and push the valves into the sockets. I do it every now and then.. Nice firm valve seating, and wipe them off.
Regards