I played a very familiar recording and could not detect what I felt to be any brightness.
It is important to note that bi amping at this frequency has the effect of removing the bloom, grit and other rubbish from the mid cone driver that can otherwise mask the upper frequencies unless great care has been taken with the full passive crossover network.
For this reason an inexperienced user may interpret the subjective result as bright. The apparent bright presentation in Mark's system could be due to a number of environmental factors.
Careful adjustment of the level controls can be used to fine tune the overall balance.
Ian