It sounds like you've identified the reason that people typically recommend bi-amping using the same amp (or same model of amp) for both low and high. Differences in amplifier design can really make for some wacky listening. I've had similar results using significantly different types of wire in a bi-wire arrangement. I'm not sure what the solution is, though, short of using either one or the other rather than both...

------------------
gonk -- Saloon Links | Pre/Pro Comparison Chart | 950 Review