Assuming that it’s wired correctly, changing the bias to V1 (that’s what the mini “boost” switch does) can increase (or decrease) noise, probably more like hiss than hum though. Which makes sense, just as raising or lowering gain can increase of decrease noise (or hiss). Changing from a 12AX7 to a lower output (less gain) tube like a 12Ay7 can decrease hum/hiss. Both of my 2Strokes are really quiet (with 12AX7s), but there is a detectable increase in “noise” when the highest gain option is selected with the “boost” switch. It is possible that when you get the chassis buttoned-up in a cabinet with a shielded cover over the exposed side you will have a reduction in noise.