What's causing hiss??

Is the hiss identical for both the left and right channels? Also, do you get the hiss when you switch to an input that has nothing connected to it?

"T-amp is 15 watts, integrated is 150 watts, speakers are a super efficient 104 db Klipsch LaScala."

That's the problem. When you have speakers that efficient, you have to be careful matching your components, otherwise you'll get a gain related hiss like you now have. Can you list your entire system?

Does your PS Audio DAC have any type of gain settings? It could be internal or external. If it does, lower the gain and that will probably fix the problem.

"Have you tried a different set of speakers with a lower sensitivity? If the hiss is still unacceptable with different speakers, then it would seem that there is a problem with the Vincent’s preamp. If the preamp does have a problem, then doing anything else makes no sense until it is repaired."

I agree. If you have the means to try a less efficient pair of speakers, that would go a long way as to finding out the nature of the problem.