You can never be sure how a tube is going to operate in a given circuit unless you try it in the circuit it is being used in.These tube vendors,including Mesa do a decent job of matching tubes to a given circuit,but its not precise and there is a certain amount of latitude taken.Even if I was to use this system,I would not be comfortable unless I checked the bias,and if I check the bias I am going to want to reset it.Matty,what happened to you is not just a case of Eurotubes not giving the best advice they can,but if they cant measure your amp the advice is an estimate,which is the best they can do.Although it is rare enough that they can sell tubes in this manner and feel fairly secure they are selling a safe product, what happened to you is always a chance you take not checking and setting the bias with each tube change.Even if the tubes fall into the right or safe range they wont always be at the best setting for tone and peak performance,this makes no difference to a lot of players and if using this system for your power tubes works for you,so be it,but you can go your whole life and never have a problem,but what happened to you is always possible and the reason I would never use them and I recommend putting an adjustable bias pot in any amp without it,it is an easy and inexpensive mod in any amp and you will always know your power tubes are performing at the best for your tone and the amps reliability.