"I hadn't heard of vertical biamping, even though I have considered that the tweeters basically don't tax an amp at all."
That's the problem. An amps power output is based on frequency/resistance. The lower the resistance (frequency), the harder the amp has to work. In a horizontal setup like you have, the amp for the tweeters doesn't have to work nearly as hard as the amp for the low frequencies. An amps sound will change depending on how hard it has to work. When you use 1 stereo amp to power each speaker, both amps are being asked to work the same amount. Vertical solves a lot of problems. To be honest, you really shouldn't make any decisions until you either do a vertical biamp, or just use 1 amp for everything.