[morzh]

OK, it seems like an X-cap improved the sound a bit.

I thought about the MAINs line contributing to the radio reception (have to yet understand how it is....since it is a pair it's gotta be a common mode interference and should be rejected...but I do not claim to understand the whole mechanics).
Anyways I have several X-caps and I put 0.68uF across the L-N, and the sound improved. Or maybe it is my imagination.
And while at that, I installed 100K 1W resistor across the AC line to discharge that cap that gave a pretty menacing spark when I decided to see how much it stored.
There is a convenient place right across the bakelite housing the two Y-caps.

I am now tempted to try the same in 37-116, 9S-262 and the rest, but they unlike 90 sound good even with that interference, so I only hear the hum when the music stops, whereas in 90 it causes the distortion. The latter is not fully gone, but seems to subside quite a bit. I am unwilling to start installing micro-farads worth of caps as the inrush will be large and the sparks will be pulled when plugging-unplugging.