Another response from Vox Support in the UK. It was pretty brief, but they did answer almost all of my questions. Here's the detail, point by point:
1. I asked them to absolutely confirm that 54 watts is the maximum power consumption for this amp. Their response: "Yes, this is correct."
2. I asked them to confirm whether the quote 120 watts RMS is a continuous figure or more of a momentary peak figure. Their response: "RMS is an average, the figure is normally quoted when a sine wave is injected into the circuit. As you are from an engineering background, you will be familiar with the math’s and methods involved in reaching this figure http://en.wikipedia.org/wiki/Root_mean_square." This response actually didn't answer my question, or tell me anything I didn't alreayd know.
3. I asked that, if the 120 watts RMS is a momentary figure rather than a continuous one, what is the actual continuous RMS output figure for this amp. Their response: "120 watts RMS".
4. I said I agreed with their earlier point that amplifier output depends in part on the efficiency of the circuit design, but asked them whether or not they agreed with me that no amplifier design is more than 100% efficient. They chose not to answer that one.
They restated what they told me in their previous email - that the VT80+ does produce 120 watts RMS into a 4 ohm load (although I note they don't use the word "continuous").
They closed the email by saying they would not discuss this issue with me any further, and if I felt they were misleading customers, I should take this up using whatever method was appropriate in my country (I'm in New Zealand). I plan to do so, as I believe they are in breach of consumer law here.
To sum up: If what they say is correct, this amp can produce a continuous output of 120 watts RMS while consuming no more than 54 watts of power. Am I the only one on here who thinks they're breaking the laws of physics here? Interested to hear what you all think.
On a side note, I was in my local music shop yesterday. They are not a Vox dealer, but they do sell amps from Marshall, Fender, Mesa Boogie, Orange, Line 6, Soldano, Gallien Kruger and others. We did a quick, unscientific survey of about 15 different amps of all shapes and sizes. Every single one has a rated power consumption that is significantly higher than the quoted RMS output, and in some cases it is three times or more. Is anyone aware of any amp builders other than Vox who are claiming more power output than power input? I know it's not just the VT80+, their figures for the whole Valvetronix range are similarly supicious.