Rested strings?
 

This is probably a question without an answer but I'll ask anyway.

I took my 4 year old spiros off my bass to try some weedwhackers for the hell of it. They were on for a week or two. I put the spiros back on tonight - and they sound almost like new - bright, nice overtones, and I like the sound (many don't, I presume). I've done this before, but after a week of playing they go back to sounding dark again, less interesting for me. Its a ply bass.

What's happening here? Is the bass relaxing or the strings? And how can I keep them sounding the way I like them? Maybe i should wind them down every night ...

Relaxing?
 

All strings stretch a bit under tension. Spiros like most metal strings have a core and many under windings as well as the cover wrap. When they 'relax' a bit when not under tension they sound different than fully stretched out. Darker sounding Spiros are preferred for Orchestral players. Maybe you can get a swap program going with a local Bowing guy. You buy them fresh and when they darken up a bit, sell them to the Orchestra guy for half price. I would love to get strings with the edge taken off when I put them on.

As long as they are not damaged, old Strings work great for the Bow if they hold pitch and intonation.

I had to put my Weichs back on after solo Doms and those Weichs sound really good now. They're probably about 3 years old!