What you’re asking is how to I determine if it’s the tube or the supply.
Unfortunately, there is no easy way to determine that. There is no easy way to test the hv out of the lps or how it relates to tube operation without specific instruments most of us do not own.
I’d suggest you check the tube resonance or mode, if that’s bad, you need a new tube. This manifests in many ways, so a quick check will tell you one way or the other. If it’s not in TEM00 mode, it will never work properly.
When I replaced my tube, I didn’t replace the lps… I don’t have deep pockets, so I prefer to troubleshoot it down to the Optimum Replaceable Unit (ORU) i.e. tube or lps. Not replace them as a set… I don’t trust Chinese replacement parts that well… might end up with something worse.
In reality, if the tube appears to be in TEM00 mode usually means tubes OK, then it’s an lps.
If not in the proper mode, change out the tube as you will have to anyway.
Hope this helps
