Much as I loathe the sound Steve Harris gets (and yet I'm a big 'Maiden fan)...I'd suggest:
- You only seem to get this effect with Fender basses, so find a bass sound that's based (pun unintended) around a P-bass sample
- Try a compressor afterwards with fairly slow attack to let any attack through, and EQ around (from very old memory) 2-4KHz
- If this doesn't work, you could do something complex like freeze the synth, use Melodyne to get the start of the notes and their pitch (drag the audio clip to a MIDI track) and add a second synth with a "click" sound at the start of each note, short duration, probably time-shifted a few samples earlier than the note start
I found this "effect" from a P-bass very hard to work with in a mix as it clashes with other instruments.