Bi Wiring should make no difference unless you use undersized wire (as @carlsbad2 said) . If it does, then something else is going on to give that effect. The two "bi wire" signals are actually full range signals coming from the full range amplifier(s), remaining full range down the two cables (what you think is "HF" and "LF" cables) then inserted to the crossover and passively filtered to remove top end for the LF feed or remove low end from the HF feed, following the crossover's plan on where the signal is destined to go. The typical benefits of bi-amping (according to the old original definition of biamping that is still used in pro) was that an electronic crossover that operates at line level can do the separation work better with less error and then each amp can specialize in HF or LF. This gave obvious benefits as low end typically draws the most power from the amp and is usually the root cause of amp clipping (running out of power), causing you to hear a clip in the top end. Clipping is usually the reason tweeters die (from overheating trying to follow a clipped square wave); the dynamics of music is not linear but logarithmic, so dynamic music peaks can instantly demand many times the output of the average, often outstripping the amps ability to supply it. This is also one reason a larger amp usually sounds better than a smaller one of the same design.
Brad