Of course, what we really want is more intensity in the sound, and bodies which are relaxed and engaged in expressive, free movement.
One of the precepts of kinaesthetics is that 'the biggest muscle wins' - that is, if your arms are making big circles you'll find it hard to sing softly. This can be used to distract tension from the upper body if the singer is encouraged to engage the big leg muscles, lowering their centre of gravity and usually giving them a much more relaxed and centralised (balanced) posture than when they are trying to be intense. The 'trying' stance is characterised by weight significantly forward onto the toes, full upper body leaning forward, 'turtle-neck' head... A centralised posture offers plenty of energy from the legs while providing a relaxed vocal mechanism.
If you pursue the kind of sound that comes from singing with full resonance and locking voices together, there is no need for 'surface tension' - whatever the volume, go for poised bodies and free sound. Beware of asking for intensity in a way which encourages your singers to sing 'tense' in some instances and relaxed in others. Be consistent in your requirements and the consistency of your performance will increase.