Tube location info: The balanced ECC83S is located in V5 for the phase inverter, which is closest to the power tubes.  The ECC81 replaces the 12AT7 in V3.

Combo amps are very hard on tubes because of vibrations which can cause tube rattle with any brand of tube. If you play by yourself in a quiet environment or record with your amp you may want to consider using the EuroDamper tube damper rings for the two 6V6S power tubes and the rectifier tube. These are available in the "Related Products" tab as a set of three and will greatly decrease tube rattle.