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I have a piston-valve horn that is three years old and I'm not as happy with the action of the valves as I think I could be. My #1 horn is a rotary and I'd like to get a bit closer to the light & fast action it has. Now that the valves have been in use for awhile (i.e., 'broken in') I think I could probably change out the original springs for ones with less compression resistence but retaining the same extension. To that end, I'm sure that some of you guys have experience in this area and can offer some guidance, as well as maybe some input on how valve spring specifications are arrived at and considerations to bear in mind when changing them.
(FWIW: The horn I'm trying to 'lighten up' is a Yamaha 621.)
Thanks.

You may have a real problem. My 621 valves were scary fast. I hardly ever had to oil them. They were much better than anything I had before or since. I did have to learn to use super light weight oil to keep from gumming them up. The tolerences are evidently very close in those horns.