Settling interconnects by shutting down your amp and running your tuner through the powered up preamp will work fine. When I am settling new cables, I simply leave my tuner on at moderate levels while I'm away and at low or listening levels when I'm home...for 200 hours straight! In fact, if you aren't using tube gear, I recommend leaving all solid state gear powered up 24/7. In most cases, this amounts to leaving a 100 watt bulb on all the time. I'm still using gear that has been on 24/7 for over 8 years, with very little down time.
If you've ever experienced those swings between your gear sounding good on some days and not so good on others, leave it on and this will all but disappear.
If you've ever experienced those swings between your gear sounding good on some days and not so good on others, leave it on and this will all but disappear.