Micro-Trains N scale couplers have probably the softest and finest springs I have ever seen. If MTL can get those made, there is hope for Protomate springs getting softer (sounds funny in a "get it larger and harder" world).
following,,, btw we make our own springs...6 different ones I believe. The set up to have something specifically prototyped is extensive and most companies will not work for what they consider low production runs. The trick is getting the right wire to start with...
Joe