Each of the HP60’s six headphone amps has its own level control and is loud enough to drive the alligators out of the Louisiana swamps. And the HP60 lets you customize each mix. In addition to the two pairs of main stereo inputs, you get an external stereo input with level control, mono, and mute for each headphone channel. So you can send each musician a feed of just their instrument or voice and can control the level separately for each.
The HP60 features two sets of line-level, stereo inputs that are routed to all six headphones and a Mix control that sets relative levels for each headphone output. So you can feed the main control-room mix to channel A, feed the click signal to channel B, and use the Mix controls to individually raise or lower the click relative to the main mix for each musician.