That is "soapbar" as in Gibson P90 rather than the sense in which the term usually applies to bass guitar pickups.
The polepieces of these pickups are filister head screws. They thread through a plastic tubular section in the bobbin and, then, through the metal baseplate of the pickup (if present).
Removing three of the screws from such a pickup may defocus its magnetic field above that half of the pickup but it will not stop it altogether. For that, it would be necessary to use shorter bar magnets below the bobbin. Even this will not create perfect separation between the wanted and unwanted signals from the strings.
What amplifiers are you using? Which ABY box? How good, bad or indifferent are the cables carrying your signals?
Does the RJT-60B sound muddy when you play chords through just the bass amp? This could possibly be the fault of the amplifier rather than the instrument.
Finally, what resistance value and taper pots does the RJT-60B use? 500k Ohm pots would be normal with the P90 type pickups. 250k pots will suck away a lot of the high frequency content - especially if the pots are cheap 'n' nasty.