Actually, with balanced push-pull amps (99.9%), clipping occurs in both directions, but that's a minor point. One should also note that speaker impedance varies accross the frequency range, often being lowest in the lower range. Therefore biamping with a higher current capable amp for the bottom end can allow you to keep your sweet tube top end (if that's what you want) with reduced likelihood of clipping problems related to driving a full range speaker with a modest amp. P.S. The power rating often associated with many mid to low fi speakers is not much of an indicator for this concern.