[Amfibius]

cmalak, I am currently bi-amping with a solid state amp for bass and my Cary valve amps for midrange/tweeter. At the moment my measurement setup has been lent to a friend but I will get it back fairly soon (I hope!). And then, I will be able to tell you how much difference in phase there is between the two amps.

In any case, I believe it is far more important to worry about making sure that the amps do not clip - and stay within the linear range of their output - than worrying about small orders of phase shift and group delay. Others may have a different opinion, and I accept that. I might change my mind after I do a few sweeps and quantify the phenomenon myself.