Tube Burn-in Process: Psvane

My question is not limited to the vagaries of tube burn-in but to the life cycle of amps, pre-amps, and cables even. I'm about 20 hours into the break-in period of a quad of Psvane CV-181-T's in my VAC Phi 300.1 It's expected that right out of the box, they will play closed-in, frigid, as it were. At about 10 hours they bloomed deliciously, but shortly after, receded again and at 30 hours still haven't regained their exuberance.

I've noticed the same close-open effect with cables and other electronics as well.

Any ideas as to why ?