I thought about this yesterday and there are three possibilities:

1 - All the PSU caps were replaced with Black Gates which have a different spec to the original caps and are slighly higher capacitance.

2 - We were facing magnetic north at the time of the test or perhaps the iron in all that Guinness we drank was having an effect on the field in the room ??

3 - As we thought on Saturday, perhaps the input impedance of the Bryston active x-overs was lowish and somehow reflecting back through the S&Bs ??

I think it's likely to be a combination of 1 & 2