I have reservations about bi-amp with different types.
First of all there are gain-matching problems. Your X250 will most likely have a different gain then your tube amp. Unless the tube amp has a gain control or you are using an external crossover with gain control, it is impossible to achieve a perfect match between the two.
Secondly, a tube amp and a SS amp have very different tonal quality. It will definitely cause tonal coloration around the crossover point. If the ProArc 3.8's crossover frequency is very low (less than 120Hz), you will be fine because human beings are less sensitive to tonal changes at low frequency. But if the ProArc has a relatively high crossover point, you may not like the results.
