Well I went ahead and hooked up the tube amp to my SS. After some research I discovered my tube pre-amp (little bear P5) is actually a buffer designed for line level. I ended up connecting it to my new Rotel RA 11 after I realised it wasn't going to blow anything up
It works, whether it sounds different or better I can't say without just yet.
Following Markw's advice above I set the pre-amp to the same volume level as other line inputs and left it there. Poutrygeist is suggesting to handle volume in the exact opposite way so I'm not sure what to make of that. Surely running an integrated amp at 100% volume all the time isn't ideal?