If I were you I'd run both amps like u were thinking... Zapco to tweeters, Diamond to speakers...then all you need to do is pick up a DSP and you're active. I just got the Dayton Audio DSP-408...for $150 it's amazing. I haven't got to change my setup yet- I have SB Acoustics SB17NRXC35 6.5s and TB25 tweeters, but right now I'm just running 2.75 Memphis dash speakers off stock HU and NVX NSP65 coaxials
I just hooked up the DSP to the coaxials and my sub just to test it and try to get familiar and just doing basic (very green when it comes to DSPs) tuning my setup sounds much cleaner. Cant imagine how it's going to sound once I get the front stage installed...
In other words I highly recommend going active & the Dayton is not just great product bc of price, it's just a great product.