right now, the only 'power springs' i can find for sale is by a ebay seller...tho not sure what brand his is.
Tremolo Power Springs for Ultimate Tuning Stability T59 - eBay (item 370059173997 end time Jul-09-08 07:48:35 PDT)
interesting. I understand old, worn springs can affect return to pitch.
A TGP member posted abt hearing a difference in sound when he swapped the springs to ones of different length and there seemed to be a general consensus that its true.
I dunno.
Maybe one day I'll try... for now, I'm using callanham springs. I dunno abt their tone but they don't seem to tarnish at all!