What's causing hiss??

"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?

There are a bunch of things going on which could create the hiss especially when using very high efficiency speakers. Component noise floor being one of the major ones in either ss or tubed units, especially the latter where the noise floor is often increased when it doesn't have low noise tubes as I suggested above. But another thing which must be considered is the gain in the pre-amp (section) and the input sensitivity of the amp (section).

With modern high sensitivity amps, tubed pre-amps must have low gain (0 to 12db). Pre-amps with more gain will (26db +/-) will usually not work so well with high efficiency speakers). Those high gain pre-amps will however work well with older amps which need the high gain to drive them properly.

The moral of the story is that your conclusion may be right and it would not be too expensive to find out (assuming that you are happy with the sound of that combo) but you might like to do a bit of meaningful research into this subject so you can understand what is going on and why as opposed to just finding out the hard way thru trial and error.

FWIW on the issue of pre-amp gain one can reduce this gain by inserting simple single resistor fixed attenuators between the pre-amp and amp section (such as are made by Rothwell). You can select the amount of gain you want to reduce when you order. If you are hearing hiss at the listening chair I would think you might need as much as 20db. Just something else to think about. :-)

Amps/integrateds on audition:

Bel Canto e300id
Marantz pm8004
Outlaw 2150
Vincent 236MK
Trends ta-10

Source: apple lossless=>sonos=>ps audio DAC

Speakers: klipsch lascala early 80s vintage, recently recapped

I:ve been trying all manner of combos using pre section with the variuous amps. So far i like the tonality of the Vincent (tube) but noise is very high.

Marantz is very quiet but a bit laid back. Bel Canto is quiet but metallic/cool sounding. Bel Canto pre is very quiet with any amp, Vincent pre is the opposite.

Used to run Khorns for 35 years. A new amp/preamp will not solve the problem. The Rothwell shunt is the easiest solution.
Had the Lascalas before the Khorns, dynamics are always incredible.
Would spend my money on Rothwells and new crossovers sold on Ebay.

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.