Threshold S/300 mods?

I own a pair of S-500s (Series I). Among the various permutations I've been through over the last year or two, I ended up with one of them running my tweeters. This wasn't exactly the optimal setup, as they're far happier in the bass region. I pulled four old 4uf 425V WonderCaps out of the junkbox and put one across each electrolytic. It made more difference than I expected--all to the good. Currently, the Thresholds are back on my woofers and subwoofers, so I took the caps back off.
Curiously, I put film caps (Solens, this time) across the electrolytics in the Aleph 2 power supplies, but heard little difference. I'm still mulling this one over, as I can see no reason why there shouldn't be an improvement of some sort. Ah well, one of those mysteries that makes things interesting...