Colin,
I believe these are the potential sources of DEC backlash:
1. Worm to ring gear interface
2. Worm longitudinal play
3. Belt gear to worm interface
4. Belt loose
5. Belt gear to stepper motor shaft interface
It sounds like you have checked 1 and 4. 2 is easy to check if you remove the cap that conceals the worm bearing and adjusting ring (you need needle nose pliers for this). If there is longitudinal movement of the worm between the worm bearings you can feel the worm move with your finger when you change DEC direction. If this is the case you need to tighten the ring that holds the worm in place. Have a look at "Stage 2 - Stripping the worm carrier" on Astrobaby's website:
http://www.astro-baby.com/EQ6%20rebu...rip%20down.htm The third image shows the ring that adjusts tension. Don't over tighten this as the worm will not move freely.
3 and 5 are more work. Did you ensure you engaged the set screws for the belt gears on the flat section of the worm and motor shaft when you installed these and that the set screws were locked down tight. I've seen cases where the belt gear rotates slightly on the shaft because the set screw was not aligned with the flat on the shaft.
If all of these are ok then it is possible you always had this backlash but you were operating on a different section of the DEC ring gear. EQ6 ring gears often have a tight spot and a loose spot however since the DEC axis never completely rotates it is possible to adjust the mesh and the position of the ring gear so you normally operate in either the tight part or the loose part. You can change this by releasing the DEC clutch and rotate the DEC axis with the hand controller.