As there are two types of flats "Old school" and "Modern Hybrid", I suggest you use both, one on each bass. The TI's are old school - thumpy, warm, smooth. Modern hybrids (Chromes, EB Cobalts, Fender flats) have a warm, brighter tone, more articulation through the mids, which may be good in your big band, will help cut through the mix better.