Ok, this is interesting. I really like what you did already. Great trouble shooting technique swapping out the tubes. I'm going to look for a schemo for this amp. I have never worked on a TLS. Off topic a little, head or combo (It really shouldn't matter)

Was the amp working well before the tube change and attempted biasing?

I'm beginning to suspect a bypass cap has failed. I'm sure you know there are LETHAL voltages inside the amp.

Let's do this, see if the multi meter will show you the voltages. Change the ranges to V instead of mV.

I'm off to find the schemo.