Low current draw in the triode. Basically anything off at pin 9 will seriously mess with how that stage balances and performs. Yours seems to be drawing so little current that I'm not surprised its not working well. Check the voltage at pin 3. The pentode is direct coupled to the triode, and the voltages at the pentode plate are strongly influenced by the screen voltage at pin 3. Verify the 270K and 100K resistors are on value. If those are good, one option would be to adjust them to get the voltage at pin 9 where it needs to be. If pin 9 is close to right, the rest of it should fall in line nicely. Trying other 6U8 or 6GH8 tubes may also yield some joy. The annoying part about those pentode/triode tubes is they are often pretty variable in terms of performance. Luckily yours happens to use fairly available and inexpensive ones so it won't be the end of the world to try others.