Any specific advice? Currently got a threshold of -13.5, ratio 5:1, attack 9ms, release 50ms. It sounds really quiet without any compression.
There's no number when you talk about compression. I'm sorry, but you'll have to trust your ears on this one. Reverb can help piano sounds too.
The attack and release sound about right. If its weak, try EQ and parallel compression, and a bit of drive. Should sort it out. And remember multiband compression works wonders on piano and strings
I don't want to start an argument here, but you can't make music by checking waveforms. How the hell are we supposed to know if a piano's sound is ok, just by looking at it's waveform?
I check the waveform all the time for everything ( I work in audio a lot), you can tell a lot about the dynamics and transients. Resampling bass you can see where to look for interesting bits or clean subby bits just by looking at the waveform. Of course you use your ears as well but just saying.
With compression referencing the waveform is essential for me. My ears aren't trained well enough to hear the subtle changes. i'm learning as i go so maybe one day i'll hear it, but until then i'm using my eyes and an oscilloscope.
Just don't compress to much as all the background noises will be louder and you loose your dynamics. For the rest, train and trust your ears.