i do not have the said version of your peakers, but i do own a f200a based speaker, which is the larger brother of the drivers used in your br set up. i drive mt mass loaded transmision lines with ss chip monoblocks...there is no sibilance so to speak. none. in fact i call i the alnico magnet sound signature, robust, with plenty of weight as well as being clean top to bottom.
check the interconnects as someone else have stated...if that is not the case, you might have to wait for your tubed amp to be rebuilt to see if it is the choice of amplification which is giving you the sibilance.
when the amp comes and you still feel that there is sibilance, then, i say check the speaker itself...sometimes, untamed backwave which is not absorbed in the cabinet can produce a peak in the response of the speaker, which will tilt the presentation upward. ah, do not forget to break those drivers in....people have recommended a good 100-even 500 hours of proper break in to really get to the heart of the alnico driver.
great drivers btw.