So, as an accident of tweeking, I started using two preamps in my system. One preamp (the primary preamp) feeds two CJ tube monoblocks (200 Hz and above). The second preamp feeds two Belles One amps (200 hz and below through an active crossover), as well as a subwoofer - the preamps are connected tape out -> line in.
The sound is amazing. Most notable is the sound of drums. Probably the most unrealistic sound my system produced was drums. A live drum kit is a loud, obtrusive, shocking, and overwhelming instrument. Played back on my single-preamp system, the drum sound is balanced and provides the necessary pace, but does not sound nearly as dynamic and startling as a drum kit should sound. Bi-preamplifying makes a drum kit sound much more realistic in my system. The other big difference is that the percussive and dynamic aspect of every other instrument (pianos, brass, strings) is now readily apparent.
So, for those of you who bi-amplify, I suggest an experiment: take that spare preamp you have sitting in the basement (you know you have one :-)), and hook it up – tape out to line in – and feed your bass amp a full repeated signal!
Were you using any preamp before at all? If you were, perhaps the impedance was too low for the first one with all the connections to the various amps.