hi oldsalty,
Watch Fishing Western Australia on Foxtel Sunday morning - they used a knife jig to downrig for samson fish
The way they did it was to have a crane swivel at both ends on the jig - the jig only 1.5metres in front of the hooks - as the jig also acts as a attractant.
Troll speeds were slow at between 1 & 2 knots with them stoping every minute to allow the bait to sink back down to the zone.
good luck