It is possible that the level of hiss that you are hearing, or not hearing, in the various configurations, has something to do with impedence matching between the sources and the amps, as well as the IC's used to connect them, when using either the Cary or the Endlers. I don't know. Passive pre-amps are problematic and improperly used could easily result in a roll off of frequency extremes. which you have interperted as a reduction of hiss.
What I do feel confident about though is that the TP is the source of the hiss and reducing the hiss at the source, assuming it is not inherrent in the design, is the best, if not only, way to skin this kitty.
The Endlers placed between the TP and the Cary will only reduce the overall output of the TP - it will not just reduce the hiss level and leave the rest of the signal intact (other than perhaps introduce some impedence issues referred to above). When you crank up the volume to the same levels at which you heard the hiss before it should still be there.
But you can't hurt anything by trying both locations for the Endlers just to satisfy your curiousity if not solve your problem. And if it does 'solve your problem', used in either location, and doesn't introduce any degredation to your ears, then great.