Biamp question

You do not need a crossover- just let the speaker do the work. 

connect the bass amp to the bottom speaker terminals and the mid/treble amp to the top set of terminals. 

you may have a gain issue that causes a miss-match- 

You would need an RCA Y-cable to spilt the preamp signal.  

PS I have owned the a21+ and the Stereo 100.  Have you tried just the stereo 100?  I found it as powerful as the a21+ and your speakers are very efficient.  Try just the ST 100 if you haven't.  

@backdoor 

"Thank you all! I believe I can manage the active crossover (possibly) but my chief concern is disabling the crossover in my speakers."

As it should be.  The crossovers do much more than dividing the frequencies.  They also may include-

1) Baffle step compensation (reduces front baffle reflected midrange and treble so as to even those frequencies with the bass, can be as much as 6db. 

2) Impedance flattening circuit.  Makes the woofers easier to drive and have a flatter response curve.

3) Notch / peak suppression filter.  Reduces the magnitude of driver break up frequencies or other frequency peaks that a typical crossover and slope cannot deal with. 

@backdoor 

agreed, bypassing crossovers can work and work well but DSP per channel with parametric EQ and knowing the driver's parameter are musts.  

DSP is also a rather large rabbit hole.  took me two years to fine tune my car's active 3-way plus sub system.