I re-read things and I see you are talking about under-powering the subs. In this case, as you have noted already, yes you are. However, 390W is not exactly trivial: even if you drive the speakers at half power, they won't be damaged by the amp. However, if you set up the amp so that it's delivering full power all the time, I would have thought this would not do much for the longevity of the amp!

Can't advise on this, as you are way above my thresholds: however, I would be setting up the amp gains so that it is *just* starting to clip at 0 dB which would set it up nicely for lower output levels.

Watch out for your ears, though - you only get one set as I am discovering to my cost.