I've tried passive biamping with Audiolab amps, and though I thought there was an improvement, it wasn't a big one. I only did it to try it, as I'm in the process of putting together an active biamping system (actually, triamping), as I've had a system like that in the past and I liked it a lot.
For that, I need an active crossover (which goes between the pre-amp and the power amps). The problem is, nobody really makes them. The main reason is they have to be set with their crossover frequencies matching your speakers, and that's not something you can adjust with a knob - it's factory set by choosing a network of components of specific values. So I'm building my own crossover based on a Rod Elliot design, and I calculate the values of the components to suit my speakers. And then, to get the full benefit of active biamping, you really need to remove the crossovers from your speakers (and then install some tweeter protection). If you don't, you still have some of the problems that active systems are meant to solve. So unless you can build a crossover yourself and can do the speaker mods, you probably can't put together an active system based on your existing setup.
But, if you want to try passive biamping, you can probably do it relatively inexpensively. As NifkinFZ6 says, you can get suitable Arcam power amps on eBay at a reasonable prices. And if you get one and try passive biamping, and then decide the improvement isn't worth it, you could resell the power amp for probably very little loss, if any.