Setting aside any possible low level imbalance in the volume pot over the very first bit of travel, with a mono signal at normal volume and the balance knob perfectly centred L+R output should be equal.
Of course, any slight imbalance between each speaker combined with any acoustic imbalance from the room can change that, and then add any slight imbalance from either source equipment or media, and perceptible imbalance can result - it’s why a balance control is sometimes rather nice to have.