Whenever I write a piano part over an orchestra (ala "The Piano" soundtrack) it always sounds like it's in a totally different sonic space. How do you get that polished, well-blended hollywood the-piano-belongs-here sound?
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How do you place pianos in an orchestra?
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These are all shots in the dark, but I think that if it sounds like it's in a different space from the rest of the orchestra, the obvious answer would be to put it in the same reverb so it's in the same space.
After that, my favorite reverb parameter (after the tail lengths and timbre) is predelay. Long predelay settings bring selected instruments forward, letting you hear all the details while still giving them a lot of reverb. It's great on strings, and it sounds like that's what you want to do to the piano.
If that isn't the right sound, a totally different thing you could try would be to put the piano in its own reverb, but use a shorter version of the same program as your main orchestral hall. But I think predelay is what you want. 45-50ms is a good starting point. Much shorter and you'll defeat the purpose; much longer and you'll hear the reverb as a slap echo.
The other thing is that a lot of sampled pianos pan each note farther to the right as you go up the keyboard. That's a great effect for some things, but it's not what happens when you're listening from a distance as you are in an orchestral setting - so you probably want to collapse the pans close to the middle and get rid of that.
In a different context, I'd also suggest the old "use hard-panned stereo delays at about 15 and 40ms to create walls and then send the whole thing to the main reverb trick," but that's probably not the right thing to do here. Or maybe it is?