I don't find too much value in having a matched pair, although I will spend reasonable money for that option if it is available. Doubling the price of the tubes is not reasonable. El84's can be had for about the same price as 12AX7 tubes ( about $15 each ). A matched pair shouldn't cost much more, perhaps $10-15 more fairly, beyond that you are getting ripped off. It only takes a moment for some person to stick the tubes into a machine to measure them. And with a properly set up system, a computer program will tell them which tubes to put with which at the end of the batch. There is nothing special about matched tubes, they are simply measured and sold with another tube that is close in performance. It takes only a couple of minutes for that match to be made, so spending much beyond an hour's wage for that is silly.
I kind of figured that the amp was fixed bias, so I am not surprised there is a bias knob in there. You are looking to bias at around 70% of the tube's peak output. So realistically you are shooting for about 7-8 watts per tube or roughly 14-16 watts of total dissipation from the pair. This link:
https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm
has a very good primer on several different methods of biasing. I too prefer the method that Rob prescribes ( the output transformer resistance method ) and find it to be rather easy to expedite and very accurate with reasonable safety. The center tap is usually the RED wire that comes from the output transformer. The primary wires of the transformer are typically BLUE and BROWN.
My years of experience have made it so that I can bias pretty much by looking at the tube. I generally start with the bias as cold as it can be. This is easy to see with the amp on ( and connected to a speaker ). I play the guitar while I slowly increase the bias. At some point, one of two things will happen, the sound will change slightly, and or the tubes will red plate. Obviously, red plating tubes are bad. I look for the point where the sound is as good as it can be while still being as cold a bias as I can get away with. I find this generally will place me in the 60-70% dissipation range. Below 60% dissipation, the sound is generally sterile or lacking in character. It will sound ok, but just not full-bodied. It will also sound thinner, brighter, and even perhaps have a little bit of grit on what should be a clean sound. When you get above 70% of dissipation the sound often gets fuller, warmer, and bigger sounding. The volume and self-noise also increase a bit. The tubes will often look as if they are running hot with visible blue emission and more heat production. It may be right on the verge of red-plating at this point. This is too hot. If you look into the tube and can see the grids glowing blue very clearly, you may still be too hot. You are shooting for a faint blue glow from the grids. Using the above method to set the ideal dissipation, you can get an idea of what the tubes should look like at normal operation. I do agree that 70% is a max ideal setting, so err to the cold side of biasing if you are not certain. A cold bias will ensure the tubes last longer, don't hurt the sound too much, and has less self-noise. I guess you could say if you are not certain, err to somewhere between ideal and too cold. If the tubes are not red-plating when you have the amp cranked and you have been playing for a while, you are ok. If the tubes even think of red-plating when you are not playing, you are way too hot.
As for the screen grid resistors, they are the ones that will be connected to pin 9 of the EL84. Typically they are 100ohm to 470ohm resistors. If they ( Blackstar ) did good, they will already be 1k or higher. There is often only 1 per pair of tubes, and in an ideal case, there is one for each tube. So you will have either 1 or two screen grid resistors. They will be higher wattage too, so should stand out pretty clearly as some of the largest resistors on the board. Generally, they range from a 3-watt to a 5-watt resistor. I doubt they used 5-watts ( large ceramic box looking resistors ) though. If they don't look burnt, it is still wise to measure them anyway. They should be very close to the stated value, if not, they should be changed.
. Even just changing out a component or two.
