Hello Amfibius,
Thank you! I suppose the all-out solution for me would be to use a high end external crossover like the Pass XVR-1 crossover not to contaminate the mids/highs, feed the output of the lows to a DEQX HDP-4 to perform speaker and room correction for the low frequencies, feed another pair of Krell FPB 750mcx monos driving the portion of the passive crossover that feed the 15" and 8" woofers (I believe cross at about 300hz, but have to check). Aprox. US$25K.
Have you tried the new Deqx HDP-4? They are supposed to be way more transparent than the HDP-3.
Cheers,
VN