Bias CAT JL2 + Tube swap 6550C = KT120

Hi all,
I just bought a second hand JL2 Signature MK2 (with the 6550C tubes).

In order to set it up correctly I need the following advice:

1) what should be the general bias of the 6550C tubes? In the manual it states 30, but I read in this forum that it should be 37, so I am confused...

2) And should the right value be 30, as the manual states, it is referred to new tubes. But my tubes are not new, so I imagine that I should bias at a lower level, right? How much lower? The tubes are 1 year old.

3) Once the bias is set, the value is not steady, but it moves (i.e. if I set 30 it goes randomly and continuously from 29 to 31 in a period of 5 seconds)

4) Is it correct that after setting the bias to 30 when the item is hot then when I turn it on the day after and it's cold it marks 38 and then after 1 hour it gets to 31-32?

5) Is it correct that after setting the balance to 0 when the amp is hot the the day after (when it's cold) it has a different value (say +2.00. And it stays to +2.00 also when it gets hot)? Should I continuosly correct it?

6) Also, every time I switch the amp on, the circuit breaker comes in and cut the energy. I have a 10A on a 230V line. Shouldn't it be enough?

7) FINALLY: would it be possible to swap the stock 6550C with the new KT120? Or there are other changes involved?

thanks to everybody.