@Phil, since you opened the regulator to look at it, does it use relays to change taps or is it solid-state (SCR's/Triacs)?
Long ago I tried a el-cheapo regulator like this one and found it unusable because of too-small transformer and the relay switching times. Every time it would make an adjustment the momentary v drop as the relays switched would cause the miner to go offline. My solution was to use a programmable dual-conversion UPS like the TrippLite SU6000RT4UHV—6kVA that is always on-line and has zero transfer time. It's in/out eff is reasonable at a measured 93% in dual-conversion mode and >96% in eco-mode.