Probably ground loop hum - common problem with unbalanced instrument cables.
Is everything power from the same AC receptacle? If your piano and sampler are in different rooms, for example, that is very likely to cause this. Try to power them from the same AC source.
I would record that with a passive transformer DI such as a Radial JDI, into a preamp - I find that transformers isolate these hum problems very well.
If it is ground loop hum, the basic problem is this: the piano and sampler both have ground connections. In a perfect world there would be zero ohms between their chassis and ground - but in an imperfect world there is some resistance, and the resistance may not be equal at different points. Electrons trying to escape to earth take the path of least resistance ... literally ... so you can get a situation where your instrument or equipment chassis is a conductor of flowing AC current trying to get to earth. Very weak, but enough to annoy your audio shields.
The other explanation i've heard is that the chassis grounds are connected electrically via the AC power cables, and also via the audio shields that connect each chassis. So effectively, you have a big ol' loop antenna connected to your audio cables, picking up the AC in the air.
It's probably a mixture of both ... because often fixing the AC power supply grounding (e.g. star-grounding in a good studio) cures the problem. Which suggests the first theory.
It's also possible the piano is close to a source of EMI such as a power transformer - try moving it and see if it changes. Similar to moving an electric guitar.
Hopefully it's not an electronic fault ... that's possible too, as parts deteriorate.
www.ebtechaudio.com