Take a piccy of yours in bits and we'll have a think..
Basic stuff: a decent piston seal - what bore are they ? What does the seal look like ? Is there much meat on the piston head for a stroke increase ?
When testing with lighter / medium/heavier pellets, what range of powers do you see with each i.e. is it slamming (sleave down the trans port / lighten the piston) or bouncing (add weight / open port) ?
What is the SCR ?
Any potential for leaks around trans port and breach seal areas ?
Are the cylinders true ?
Are the piston skirts a snug fit in the bore ? If not bearings/buttons may help.
Increasing the spring is about the last thing I consider on tuning any gun
I'm guessing these are all just the same basic design as the HW45 and moderish tempest right ?