1) 2k2 is the proper cathode resistor value. 220R cause very hot biasing, low plate voltage and poor tube operation.
2) Assuming the new Celestion speaker gave rise to the feedback issue, you might have:
- Acoustical FB caused by higher efficiency of the new speaker. To compensate for this you may reduce the reverb recovery level by adjusting the 500k trim pot in series with the rev depth control to a higher resistance.
If the results are not satisfying, you would need better acoustical insulation of the reverb tank. Does it help to unmount the tank from the cab and place it on a pillow ot the like?
- Or electrical feedback (less likely). For a test you may try to reverse speaker phase and/or ground the speaker frame.