Can anyone tell me which are the fuses and values (and size) to replace with premium fuses in the BHK300 and MKII?
I do not know about BHK300, but for MKII, the 5x20mm slow, 1A is the one to use.
I am not sure if you are asking recommendation on fuses. I found AM M-1 works great for MK2, of course the Master fuse may work even better.
There are four rail fuses and the two additional ones (one for the tubes and one for the main power supply, IIRC) in the amps. The values are listed in the manual; note that some of them vary depending on the voltage of your unit.
There was discussion here sometime ago about which ones to replace if you wanted to experiment with aftermarket fuses. Perhaps someone remembers the upshot of the conversation– I don’t.