Aleph 5 passive vertical bi-amping?

That really depends on the spkrs rather than the amp. I'm sure the Aleph won't mind. But passive biamping is a bit of overkill -- no? Why not use a passive line level xover instead? Far better results there...

Speakers are Sonus Faber Grand Piano floorstanders. They are rated up to 200 watts, so I don't think there would be a problem there. Was hoping to get some extra benefits of using the 2 amps, ratehr than having to worry about tuning crossovers to start with?

Well, you'd be doubling the power (that's +3db /side). But how do you propose to do that -- unless you disconnect the woofer from the internal xover & introduce an outboard low pass. Asaik, they don't have double binding posts (i.e. "biwire"). Cheers

My GPs are the original mk1 with the bi-wire posts. Therefore I was just planning to use one channel for bass and one for treble, keeping the internal crossovers. At the moment my preamp would need Y adapters (XLR) to separate the signal to the 2 amps, unless I go for a preamp with 2 sets of outputs to enable bi-amping. I've never done this before, so I hope I haven't overlooked anything! Thought it would be an interesting experiment that should yield some benefits to make it worthwhile?

Try driving the bass with any old ss amp at hand (with sufficient output)and check out the result -- before you actually buy an Aleph. Your pre would have to be capable of driving a more difficult load (two paralleled amps). I expect there wouldn;t be a problem.