As collaborative pianists, we can feel confident about balance without depending on someone else to assess the situation.
Relying on someone who is listening from the house is iffy ~ 3 different people will give you 3 different opinions! Depending on which instrument a person plays, s/he will hear differently.
Following a recent concert, two audience members came backstage ~ one a string player, the other a pianist. One said the piano was too loud. The other? Just right. I didn’t think it could have been both at the same time, so there’s your third opinion. (The music director told me it was great.)
Performing with many different instruments and voice types has helped me figure out what’s what. Playing in a number of halls has also provided the opportunity to find out what works.
Be aware that curtains, wall hangings, carpets, seat cushions and clothing all absorb sound. Beyond that, the sound changes a great deal from an empty house to one with an audience.
Balance in general
When the piano is behind the other performer, that performer will be facing the audience. That means that their sound is headed away from you. If you can hear them, you aren’t too loud.
Many people, upon seeing the lid at full stick, will tell you the piano will be too loud even without hearing it first. Actually, having the lid open results in a clear piano sound which is easier to hear and adjust.
Several musicians with whom I have coached insist on using full stick.
Sometimes I do use 1/2 stick. I also have a “piano block.” It’s just a piece of wood, about 6″ x 4″ by 2″, painted black. In churches, especially, the block is useful. It can be set between the lid and the frame three different ways, opening the lid to any one of three dimensions. And it makes the use of a bible or a hymnal unnecessary. (Members of the congregation often find that to be sacriligious. If you want to be invited back, don’t do it!)
Range matters. When playing for flute, for example, treble sounds in the piano compete. Treble instruments and voices need less piano treble and more piano bass.
With cello, the range can be low bass or in the middle of the bass clef. Pianists need to be sure the cello is heard, and adjust the piano’s voicing accordingly.
One exception to both of the above would be when the piano has the melody or a counter-melody. When the other instrument has the accompanying part, the piano is more prominent.
When the piano has a fugue subject or counter-subject, the two instruments should match.
The Debussy Cello Sonata is another great exception. When both instruments match articulation and volume, the result is stunning! It becomes impossible to hear which instrument is which. Incredible.
It can be helpful to record dress rehearsals and concerts for our own information. I find that it’s important to listen for a comprehensive take on the sound. But the playing takes precedence, so if I start to become obsessive about the recording, I turn off the machine.
When to ask
The “system” may break down in unusual situations.
When a hall is exceptionally live, you will want to ask whether the sound is clear. You may need to adjust your pedaling and find a slower tempo.
If you have a bad cold that affects your hearing, feedback can be reassuring.
So listen to what’s going on in every situation. Process the feedback you get from others, including those on stage and in the audience. Take what helps, and put the rest “under consideration.” To gain more experience, playing in master classes is invaluable. Play for them whenever you can.