I just bought the Blackstar HT20R MKII two weeks ago. It’s still under warranty. There is a rattle with the tubes. How do I imitate a warranty repair? What are the stock tubes in the amp? Please be very specific and detailed! I know there is 2x ECC83 tubes and 2x EL84 tubes. Brand type of tubes? If tubes are replaced does the amp have to re-biased? If so. what is the setting for that?