Re: STK-S4 users
I have an STK-S4 in the middle position of my Strat. It isn't bad, but it could be better. The highs are always either too much or not enough. I love the pickup for all clean applications except any kind of blues. It does reggae and funk very nicely (and I imagine Jazz would be no problem either). Any use of my wah pedal works best on this pickup on my Strat as well.
There seems to be something missing when used with overdrive though. It's not full enough.
It does somewhat retain the Strat quack, but it's not too much (which I really like about it, I bought the SSL-1 for the vintage tones).
The bottom line is I've only had it for 3 weeks now, so I'm still learning a lot about it. I am sure someone else could probably answer your questions better, but I thought I'd kick in with what I know so far...