I like all my basses to sound the same, and with my passive peaveys, the brighter the strings the better! I like to have as much of everything as possible and if I need less, roll down the treble a bit. It's easier to take something that is there away, than to put something there that isn't! So I always use stainless steel strings.
FWIW flats sound beastly on a MM bass. Never heard them of a Warwick but you should try it! I always thought they were a bit 'hot' anyway, maybe some nice flats will tame them