I'm ovbviously coming to this late but it's an issue also of interest to me. I have a BAT VK 500 that I am exploring using in a biamp mode with a suitable BAT tube amp. After asking BAT directly, I was told by them (as others have pointed out here) that GAIN matching rather than input sensitivity was the key to making this work for a horizontal biamp set-up. The other point raised was to consider matching the general power output of both, that is, going for a powerful tube amp to match the powerful VK 500.
BAT also mentioned that inline attenuators could be employed if there was any perceived inbalance in the sound but I was told that this was probably not necessary using two BAT amps.
BAT also mentioned that inline attenuators could be employed if there was any perceived inbalance in the sound but I was told that this was probably not necessary using two BAT amps.