Velocity adjustment should be flush with the back of the hammer not fully extended. Since the SL68 takes stock nelson valve springs but Tippmann specific mainsprings. MY next step would be to clip 1 ring off of the valve spring to lighten it up. Then hope you are high and dial it back with the velocity adjuster. Or get a cocker spring kit and use the lightest valve spring available instead of cutting the stick one.
Are you using CO2 or high pressure air? A Tippmann tech told me the valves are designed for and work best on CO2.
If you are using the stock barrel it has a huge bore. Check your paint matching. Take off the barrel and feed one ball into the back of the barrel. Based on your velocity the paint you are shooting will roll through with little resistance. Your paint is rolling partway down the barrel before you pull the trigger. You will not get the most energy from the expanding gas to propel your paintball. Thus the consistently low velocity.
If that is the case you can either buy bigger paint or purchase an aftermarket barrel with a smaller bore. Personally I had my stock barrel freak bored.
Keep us posted. I love shooting my SL68II. They are beastly little pumps.
I am using CO2 my paint to bore match isn't great but it's not terrible either, unfortunatly where I play I have to use field paint so I am limited on my paint options. I have a few different springs laying around that I can play with I will give that a try and see what happens and report back.