28VAC will yield close to 38V at the first filter cap and slighly less than that into the regulator. The design spec is for 32VDC into regulator. You are likely to have aroune 36V. This is quite a lot more power and it is probably correct that the small heatsink there now cannot do the job.
It's hard to say about the distortion. I don't recall hearing about this type of issue before, but a tube change is a good thing to try. However, I suspect that this will not fix the problem I wonder if this could be related to some of the issues you had earlier with hum, etc?
If not the tube, we'll have to inspect the buffers to see what their operating points are.
To make a start, measure the voltages on all of the tube pins with respect to SG. And measure all of IC1 (buffer opamp) pins with respect to SG too. This will tell us a lot. Be careful not to short anything. As you know replacing parts is a tight squeeze.