Josep,
Get yourself a number 2 allen key and take the 7 screws out of the case. Remove the cover by sliding it back a little. Carefully remove the tube so that it does not rock the circuit board to which the socket is attached. Then change that Tesla tube. You will notice a welcome improvement with either of your replacement tubes. Try them both to see which you prefer. Also, that amp doesn't really warm up for about 25 minutes, which you may have already noticed. Enjoy!
Get yourself a number 2 allen key and take the 7 screws out of the case. Remove the cover by sliding it back a little. Carefully remove the tube so that it does not rock the circuit board to which the socket is attached. Then change that Tesla tube. You will notice a welcome improvement with either of your replacement tubes. Try them both to see which you prefer. Also, that amp doesn't really warm up for about 25 minutes, which you may have already noticed. Enjoy!