I would be careful with the McCormack 500, not because it's not a good amp. It's very nice sounding, but the input impedance is very low at 10K balanced (5K per leg) which is a problem with the Vandersteen M5HP high pass filters. Also, that circuit is not truly differential, which means you would probably get not benefit and potentially a downside to running it balanced XLR. I'm a Vandersteen dealer and have a customer right now who has that amp and it is not the best match for any Vandersteen system that requires the high pass filters.
In general, stick to amps that have higher input impedances for best compatibility and sound with the Vandersteen high pass filters.
In general, stick to amps that have higher input impedances for best compatibility and sound with the Vandersteen high pass filters.