What's causing hiss??

Is the hiss audible at your listening position or just when you put your ear up to the speakers? Assuming the latter, this is not uncommon at all and much depends on the efficiency of your speakers and/or the noise level of your tubes. Get some new tubes tested for noise, i.e. they are graded as 'low noise'. This will solve most of these issues unless you have very efficient speakers that are revealing the noise threshold level of your electronics.

Yes the hiss is the same in both channels and same volume (can hear it from listening position=unacceptable) with any of my 5 inputs selected at volume=0.

I tried an experiment this morning. I sent the tube pre section out to my el cheapo but zero noise t-amp and the hiss went down considerably. T-amp is 15 watts, integrated is 150 watts, speakers are a super efficient 104 db Klipsch LaScala.

Maybe the answer is find a low noise, low gain tube pre amp to pair with my T amp.

"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. :-)