Alexv- Unless your amps have identical gain and sensitivity, your frequency response is going to be skewed. Anyone that understands freq. response will understand that it will be skewed in favor of the section receiving the higher gain. IE: If your mid/high section's amp has higher gain/greater sensitivity than your low's- You'll end up with a "too bright" or "thin" sounding response. It takes very few DB to sound wrong, especially when the disparity leaves a notch at the cross-over point. Find out the power output of the tubed amp you intend to use, and it's sensitivity(IE: 100W output @ 1.75V input) and find a SS amp to match, one with adjustable gain(a lot of studio/pro amps have that feature), or obtain a passive level control to attenuate it (http://www.creekaudio.co.uk/products/product_downloads/instructions/obh22.pdf).