Endler stepped attenuator w/Cary active preamp

If the hiss is constant it is in your pre-amp. If it fluctuates when you use the volume control then it is in the TP. If it is in the pre-amp using an inline attenuator, either a constant fixed value or one which can be used as a stand alone VC will reduce the hiss below audibility, but depending on your expectations and listening skills you may note some minor degredation, especially if you have to use an addition set of IC's. I would prefer a fixed value resistor (like a Rothwell, but not necessarily one).

If the hiss changes with volume it is likely because of a noisy tube(s) in the TP or you may well have a noise floor issue. If the noise is the same in both channels it is likely a noise floor issue. You might have to plug in some new low noise tubes to ascertain this unless other owners confirm the noise floor issue.

FWIW.

You would put the Endlers between your pre-amp and your amps. I suspect you will discover that the noise still exists when you do.

If you are concerned that the Cary might be the source of the hiss, simply turn your source off leaving the volume control in the same position that revealed the hiss. Once again, if the hiss goes away then you know it is not being introduced by the Cary.

Topographically speaking, the pre-amp amplifying stage is always at full power. When you use the volume control all you are doing is attenuating the incoming signal from the source before it gets to the pre-amp amplifying stage.

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.