I've only used the amp on it's own, without any OD pedals, so maybe that's why I'm not hearing too much difference when using the voice switch?
As regards the cooling issue, when I spoke to Dave on the phone on Friday he said he was playing around with CPU cooling fans and that he found 1 he thought would work in the amp.
I picked my amp up yesterday, and when it's switched on with all volumes at minimum, there's a whirring noise, so I'm guessing that it has a fan in it :)