Why not E-mail Dennis Had and get his opinion. He is a talented designer who is also approachable. I hope you will share his opinion with the Audiogon community.
If I recall correctly, the tube rectifier in your amp is bypassed with a solid state device as well. This is making me wonder if its snubbed properly. If not, it could certainly contribute to hum. Tube rectifiers on their own don't need to be snubbed.
The term 'snub' refers to a network, usually a resistor and capacitor, that prevents the capacitance of the rectifier from interacting with the inductance of the power transformer in such a way as to form a tuned circuit with a resonant peak. If that peak occurs, the rectifier will make noise. Its a bit more complicated than that but that's it in a nutshell.
^^ I'm doubting the filter cap failure theory, but you might be on to something; on reading the specs on the 274b one of the claims is lower hum, provided the power supply is built in a certain way, which is to say a fairly low filter cap value for the first filter cap.
If that's true it might explain this.
When the amp showed up with the JJ 5Y3S (10 months or so ago) I thought it was surprisingly quiet, and I had tested the subsonic noise by simply touching one of my sub's driver cones…then more recently I noticed the audible low noise and sure enough, the sub drivers had a vibe goin' on. That may have been coming on gradually…not sure to what degree as I hadn't notice it recently until it became audible.
@wolf_garcia most probably unrelated to the different voltage drop of the 3 rectifiers as the 5Y3 should have the largest, but just have to ask...
You recently bought a pair of Klipsch Heresy III, with (I think, but could well be mistaken) significantly higher sensitivity than what they replaced. Sometimes, we don’t immediately notice such an impact, but any potential correlation?