If there are mono amps of which you can afford four, you can bi-amp with mono blocks. But not all speakers benefit from bi-amping, which has more to do with the speaker's crossover than the amps. And not all speakers CAN be bi-amped. Many bi-ampers like to use a tube amp for the midrange and high-frequency driver(s), and solid state for the woofer(s), while others like to use identical amps. Do you plan to use an outboard electronic x/o? Bi-amping is more involved than just using two stereo power amps.