Well the empeg specs 11-16V on the back. Seeing as the empeg has a switching power supply it's pretty efficient so in my opinion the supplies we got were just crap. The extra volt at 1A would be another 1W of power though I think the main problem with our supplies is they simply sag too much when a spike of current is required.

Perhaps try and look for a switchmode (it will be much smaller and lighter) rather than a linear reg version. Quite often they aren't even regulated and simply have a bridge rectifier and a large filter cap in them. Particularly cheap and nasty if you ask me.