Undertow, you raise an interesting point about the relationship between line voltage and bias. I have high line at my house and its something I've taken into account when adjusting the bias of my amps. I don't understand though why gear should sound best between 115-118V when its designed to operate on 120V. In stock form, final bias readings would be set at the factory with line voltage held firmly at 120V, and obviously the amp should perform best at that voltage, not something lower.
Regarding the balanced isolation transformer, I don't understand why you would get a 5% loss just from going through it unless it has been designed that way. I have worked with quite a few different isolation transformers of various brands over the past few years. All of them in stock form start out with some percentage of no-load boost, not loss. I have ordered transformers and specifically asked that they be wound without the boost, to compensate for the high line at my house. A decent, appropriately sized transformer with good regulation should not lose 5 percent under load.
Regarding the balanced isolation transformer, I don't understand why you would get a 5% loss just from going through it unless it has been designed that way. I have worked with quite a few different isolation transformers of various brands over the past few years. All of them in stock form start out with some percentage of no-load boost, not loss. I have ordered transformers and specifically asked that they be wound without the boost, to compensate for the high line at my house. A decent, appropriately sized transformer with good regulation should not lose 5 percent under load.