If it is free flowing after a properly performed overhaul and the IP has been ruled out, then yes the spring is one troubleshooting option. Not a lot of choices on the very basic design. Only other thing I can think of is replacing the adjustable orifice if it has a damaged edge.
On your 1st stage, it sounds like it had been serviced with the newer poppet valve, either current ACT or previous Tri-material. When the original poppets were replaced by the Tri-material, the HP seat was supposed to be upgraded, that was probably skipped in the past. With the old seat the Tri-material would have been troublesome, and the newer ACT would have been worse.