Is it best to use a 5-channel amp for biamping??

If you're asking whether bi-amping (active x-over, or passive line-level xover) a pair of speakers is better than driving them in passive form -- you bet.

If I understand the question: you're considering a 5channel amp in order to use 4 of these channel to bi-amp your speakers?? If so, great -- but can you disconnect the speakers' existing x-over and devise another one to precede the amps???

Greg is right on the money. Increasing power potential is always a great thing, but if you can shorten the signal path between the amp and the speakers by removing passive components in the crossover, that will get you even better results. While this would also require some type of crossover between the preamp and amp, the benefits of this type of installation have to be heard to be believed. Sean
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But I think he's asking if he can just split the signal into 2 channels to drive a single speaker in bi-wire fashion without doing everything descriobed above to the crossovers...I am curious about this myself.

I meant could he split the signal and use 2 channels to drive a single speaker in bi-amp fashion, sorry.