Bi-Amp Combo opinion

Is there any hard-fast rule about Bi-Amp combos. How about pairing an older Levinson w/ 250 watts SS for the bottom end with a recent TAD-60 60 watts tube for the mids and highs?
That should probably work fine (although I don't know what speakers you intend to power). The level control on the TAD-60 should make level matching easy.
I did something similar with No333 and Audio Research.
The slew rate is so different between the two amps; the speakers lack coherency but it has the tube charateristic.
I switched back to bi-amp with identical AMPs