On vs Auto doesn't really make much of a difference in terms of wear on the amp. The amp is pretty efficient so the amount of power draw and wear largely depends on the amount of output it is putting out. By less than midway, do you mean 11-11:30 position? If so that may be a bit on the high side, as the gain control on the STF-2 is pretty sensitive. However, depending on the material and sensitivity of your speakers, it may not be. Hard to say. For example, if you're speakers are over 100 dB sensitivity then you have to turn the sub up more to get the same output as you would with a speaker sensitivity of 90 dB. Also if your preamp is putting out a lower voltage or your material doesn't have a good amount of bass content, that may be OK as well. You may want to consider getting a larger sub. If the size is an issue, the ULS-15 MK2 is a good compromise on size and performance.