You need to find out which component is causing the hiss. First disconnect the preamp from the amp and short the inputs to the amp with the amp off. Then turn it on. If you hear the hiss, it is in the amp. If you don't, connect the preamp to the amp and short one of the preamp inputs, select this input with the selector. If you hear the hiss, it is in the preamp. If it is quiet, the hiss is in the source.
Next determine if one channel has more hiss than the other, if so, move the tubes across, channel to channel in the offending component. If the hiss changes channels it is one of the tubes that you moved. If it doesn't, you may have a cold solder joint or the like. Some designs are just more noisy than others, but most don't really have much audible hiss at the listening seat with moderate sensitivity speakers.
Next determine if one channel has more hiss than the other, if so, move the tubes across, channel to channel in the offending component. If the hiss changes channels it is one of the tubes that you moved. If it doesn't, you may have a cold solder joint or the like. Some designs are just more noisy than others, but most don't really have much audible hiss at the listening seat with moderate sensitivity speakers.