Les,
I'm not certain why this happens and, frankly even when they are very played in, the arco tone isn't near the quality of the Evah Parazzi's. The differences between Spirocore Weich and Spirocore Mitten's are the gauge of each string, and the softness of the core. I used Spirocore Mittens about 6 years ago, and if I remember correctly they produced more sound from my bass, but were harder to finger, due to their larger size. As I have smaller hands, Weich's were easier for me play on. I've been told by luthiers that the reason they are not as responsive with the bow, is that their is no damping material (silk) between the core, and the outer wrapping.
Ric
Ric-- I didn't make it apparent that my question was about the notion that Spiros lose their pizzicato sound after two years. That's what I was saying sure doesn't happen in my experience with Spiro mitts. I'm quite well aware of the differences between Spiro mitts and weichs in terms of gauge and softness. As for EPs, you may recall my extensive posts about the EP regulars and weichs. Been down that road... I love the sound and arco "ease" of EP weichs. What drove me from them was the front-end stiffness. The last time I played Spiro weichs they just felt too weak to me. Different preferences, different basses, different hands. Strings-- YMMV!