main drivers - Monacor SP-202E (10.3980) 8" dia 50W rms 4 Ohm - SPL 91dB. A compromise on size, price & efficiency - in these applications speaker efficiency, which can vary greatly, as does the useful audio volume.
Tweeters - cheap piezo horns, wired in with a series 80 Ohm resistor.
I made simple metal hook brackets to mount the speakers at an angle on the pannier frame (visible below), one side more or less following the line of the frame down tubes, to give plenty of foot clearance when cycling.
These gave loud, clear sound, coping well with bass. How loud everything went before distortion was nicely surprising, certainly more than loud enough to make a bit of a splash on central London streets.