Graphic EQ in the FX loop of the 6505.
A Danelectro Fish n Chips pedal might do it for $30 with its 8 bands. But a rack unit might get you better resolution.
Meanwhile, a solid state amp won;t care about piezos, but your 6505 will not be happy driving only piezos. It wants a proper load.
So another idea might be to make a dummy load with a crossover for your horns. You could still use piezos, or at that point evn a compression driver and horn.