An amplifier, or any other input device, does not have to be powered-up to provide a load termination for the interconnect cables driving it; signal current will flow regardless. You can prove this to yourself by measuring with an ohm-meter; the load impedance doesn't change when either powered on or off (don't DC your direct-coupled power amp if you perform this test - very bad things will happen). If you don't have an idle component to plug into for cable breakin, then you can make dummy loads with cheap RCA (or XLR) female connectors from Radio Shack. Just solder 10k-ohm resistors, or 47k-ohm, or whatever load you desire across the jacks - simple. The lower resistances will draw more signal current, theoretically accelerating the breakin. However, you cannot use a preamp's inputs to breakin this way, unless that particular input is selected on the input switch; (ie: if you're plugged into aux 1, then you must select aux 1 on the input switch) otherwise there's no load terminating the cable & no signal current flows, so no cable breakin is occuring. Regarding AC power: for best sound, never power down a solid state component if you don't have to. Reduces thermal cycling, theoretically increasing equipment life, & definitely increasing your electric bill. DO pull the plug if a thunderstorm is nearby however. Provide for integral transient-protection in your power conditioning equipment, for when you're not home during those storms. Tubes don't follow this always-on rule though.