The bias switch is only to increase the bias to a preset level when switching to the EL34's, basically it's switching between resistiors to increase or decrease the bias , the question is how much? too much? not enough? Can't normally tell unless someone checks it, if it's not set right it can drastically reduce tube life ( Remember you are buying 4 EL34's, not cool if they only last a month), weak sounding amp, hum, dirty sounding, weird distortion, it's just good sense to get it checked, you are protecting your investment and making sure the tubes are all good and the amp will function like you want it to. Buying Tubes off the internet will probably save you some money, but if you have any guitar dealers around that carry tubes it might not be a bad idea to talk to them see if they have any reccomendations, compare their prices, just be aware they will probably only reccomend the brand they carry, the best of the best is to find someone that really is a tube amp specialist, that has some years in the business and a good rep. Hey ever been to Yazoo City?...