First of all, will under-powering hurt the amp? Secondly, I opened it up and noticed that the transformer in this pre-Fender SWR Workingman's 10" is a "Topmark" brand transformer, and is labeled as "120v 100v 0v." Furthermore, there is a brown wire coming from the transformer that is uncut and has not been used. Would replacing the black lead to the power switch with the brown rig the transformer to 100v?

Answers to these questions would be appreciated! Cheers.