BBC speakers are certainly designed for use with grilles on, the LS3/5A sounds wrong with them off even to my elderly ears. I also think a lot of the ‘brightness’ some complain about with vintage Tannoys is hearing them away from their design intent (rear-mounted into cabs with grilles). Kans certainly sound better with the grilles on. That said I think I prefer my Spendor S3/5R without, and have with various ProAcs etc in the past. I can’t hear foam grilles (Isobariks, SBLs, JR149s etc) so always leave those on. As such I go on a case by case basis, but it is certainly unwise to assume off is better. They are often part of the design on older speakers.