Things to keep in mind:
HST is not the 15/10/5/negatives method, that's one implimentation of HST. HST is a set of principles with a good solid backing in science. One, load the muscle. Two, progressively increase the weight. Three, frequency, workout every 48 hours. Four, rest, or as it's known in HST Strategic Deconditioning.
As someone else said, HST is geared to size gains, not strength gains. Strength comes, not as fast as with principles dedicated to increasing strength specifically.
Also, almost all weight lifting programs adhere to some extent to all four of these principles. If you're comfortable with what you're doing and getting gains, changing makes no sense. But, if you want a change up try to modifiy your existing program to fit those four principles.
HST with AAS is different. There's plenty written on the subject on the HST boards, go read a bit over there. The stuff by the users Blade and Lars are particularly helpful, search for their posts.
The 15/10/5/negative rep scheme is just a one size fits all implimentation of HST. the reps don't really matter to be honest. They just provide a useful contruct for keeping track of volume and time under tension. You could as easily go with 12/8/5/negatives, or do a different number of reps every week. Doesn't really matter. You want steady or increasing volume and increasing weight over the course of the cycle. The greater the range of weight you can use from beginning to end of a cycle, the better the results, all else being equal. Don't be afraid to start lower than you'd think would be useful.