I wouldn't do the STS thing. Depending on how old your bow is, it probably costs more than your bow is worth. Nor would I suggest leeches, considering two sets of catwhiskers aren't quieting the bow enough. Leeches are the 'in thing' but they really work no better at silencing than catwhiskers, so you'd be wasting money. You could try yarn puffs or tarantulas. Those are the only silencers I've found to work better than whiskers. Of course, they're heavier than whiskers and will cut your arrow speed some.

You don't say what bow you're talking about, what the draw weight is, or what your arrow weight is. That kind of information would be helpful. But as a rule, if you're shooting the bow at the low end of it's draw weight range it will be noisier than if you were shooting it at the peak end. Also, the lighter your arrows the noisier the bow will be.

If your old bow is set up with dual cams instead of round wheels, you're just going to have to suck it up and accept the noise. NOTHING will make those old cam bows quiet. Back in the late 80's/early 90's, some of those bows sounded like you were shooting a .410 bore shotgun. You can reduce the noise by shooting heavy arrows which, again, will mean slower arrows but more penetrating power! If you're shooting carbon arrows, get some Carbon Express weight tubes and try weighting them down some.