It really isn't a 'power-save' mode, to be precise.
Besides some sub-miniature tubes (*those tiny one, like those in the TU-HP01) that are designed to run on room temperature, most tubes need to run on fairly high temperature to avoid 'cathode poisoning'. It is actually easy to make an tube run colder, as it is just a matter of lowering the voltage. But doing so will cause the cathode to run in sub-optimum condition, causing faster degradation and eventually lead to early failure of the tube. Of course it might take sometime to reach that point of failure so some manufacturers might choose to compromise and make their portable tube amp run colder, knowing that by the time the tube fails, the warranty will likely have expired or the amp is no longer in use and they should come out clean. While this is not exactly a good practice, it does give a more portable design. The designer of BL-2 however chooses to run the tube in its recommended voltage, thus it is hotter than most portable tube amp out there.
Edited by ClieOS - 9/28/13 at 11:28pm