I used to use a Korean Spector bass and could get a very similar tone through my Ashdown ABM.
The key was to pick the bass hard, near the neck ( if it's hanging as low as Rex's, you'll have little choice! ), boost the bass and treble on the bass and bring the gain up enough to get that 'choking' effect on the attack. Hard to describe, it sounds as if the bass is clearing it's throat, but in a good way.
My Spector has sadly moved on but it's still one of my favourite bass sounds, probably the ultimate metal tone IMHO.
