So I've been playing my P155 for a while now and I really love it, but a few of the instruments (the electric pianos and the wood bass), are very aggressive when played hard. This is great in some circumstances, but often I would really like to just turn off the top (or top two) sample. Is there any way to just pick a sample and make the piano play that?

You can, of course, change your touch setting: setting it to 'hard' makes it harder to reach that sample, and setting it to 'off', of course, makes it impossible, but also removes all touch sensitivity. I would like to use a single sample and just scale volume at times.

I think what you are asking for is custom velocity curves. The P155 does not have that. To get what you want you've need a computer in the loop. Either to modify the midi velocity and send it back to the P155 or to play a software virtual instrument.

Some DP's or other keyboards (more expensive ones maybe) allow you to setup a split/layer configuration where you can edit it so that the 1st instrument is active only within a certain velocity range and the 2nd instrument is active for another velocity range (commonly only for the remaining or slightly overlapping velocity range). This might be commonly used to bring in a second instrument when you press harder.

Not sure if the P155 has this capability but if so you may be able to leverage it. I know the P155 can do 1 layer (or dual voice) but every vendor chooses how much editing flexibility they give you for each instrument.

If you have this feature you might be able to fake it out somehow by choosing piano across full range but with velocity only from 1-120 or so, and then mute the second instrument somehow or use the same piano sound and choose velocity 0-1 etc.

Otherwise, like Chris A said you will need to loop out to a computer of some kind to do this processing and come back in.

The P155 does not have this feature.

The best you can do (without a computer) is adjust the key sensitivity to "hard" so that you will not play the louder notes then be careful and play only in the range of pp to mf. If pp is not loud enough, it's a digital piano and you can make it as loud as you want (or can afford)