I would say 500K since from what I can understand is that the Stacked pickups are basically small humbuckers that are stacked instead of side by side....Correct me if im wrong.....I have the Custom Stack Plus in my ibanez with a 500K and I really like how bright and clean it is....It is in the "Unloved" middle position, but I use it more than you think..
Honestly it's going to sound great running off either, I would suggest going with what you have already, if it's good then it's good. If you get it in there & you decide you want a little extra brightness add a 500K volume pot and you'll be sitting pretty. Unless you plan to use a full size humbucker in the bridge, then I'd go with 500K's.
These single coil stacks were designed with stock harnesses in mind so you'll get a nice sound with 250K's but I'm going to have to agree with Mr. Squirrel's comment about the 500K pot adding some wonderful brightness to the tone. I've got one running off a regular old 250K Strat harness as a bridge pup with two Texas Antiquity's in the middle/neck slots & its one of the best sounding Strat guards I've ever built!!!
On the other hand, I've got another Custom Stack Plus that's being used as a neck pickup with a Pegasus in the bridge in my H/S Strat. Its just running off a single 500K potentiometer, so about as wide open as possible & it also sounds quite beautiful. That's also one of my all-time favorite guards but they are very different guitars! The sound I get from the neck is pure magic! As is the middle position on the switch using both pup's together! I.E. Pegasus + STK Plus = Greatness!!!
Thanks for the responses. I am replacing the bridge pickup on my strat. I currently have ssl6 that I am replacing with aps-1's single coils in the middle and neck. Eventually I want to go noiseless in all positions. I will start with the original 250k pot, but would like to go to a push/pull pot and split them when I get all the pups replaced.
Yeah, I would start with 250k, as all singles were designed to just drop in the guitar with no other changes. If you need it to be brighter, you can bump up to 300k or 500k. But use what you have first.