My last of the Japan metroline Will Lee is totally noise free on all settings and even with just one of the (single coil) pickups selected. Judging by these comments certainly makes me wonder whether the Warwick pre is different. It makes sense that they may be using another source.
I have a 2014 Japanese Metroline Will Lee and depending on conditions and my position, the single coils hum like crazy when solo’d. Both together are dead quiet. I quite honestly have never seen a Jazz Bass with SCs that does not do that, so consider yourself lucky.