Interconnect Break In?

Your preamp doesn’t need to be on. But the CD player or streamer should be on and playing to pass the signal thru the cables. Usually between 100-200hrs for the cables to settle. Use a streamer instead of spinning discs 24/7 to avoid unnecessary wear on your transport. 

It’s never without you adjusting to new sound but it’s not the only component involved here. Allowing the cable to settle is also important  

Try making a change, listen for an hour then let it all play and come back the next day. Evaluate. Then leave it playing for a week before you listen again. If you’re capable of hearing the difference between the cables you will most likely hear the difference between the initial listen and when the cable had settled.

Even a used cable that’s been out of the system for some time needs a few hours of play time to settle. 
 

Rule is though, if you absolutely hate the sound with the new cable right out of the gate, no amount of break in will fix that. 

@ddrave44 the preamp does not need to be on for the cables between cdp and pre to break in because the signal is flowing from your cd player or streamer and passes thru the output interconnects whether your preamp is powered on or not. This is the easiest set of cables to break in. 
 

@ghdprentice a note on your approach.
The preamp to amp signal, unless you’re cranking it up, is much lower voltage than source to pre which is typically 2v rca and around 4v xlr. Running preamp at low volume to break in the ICs between pre and amp will take slightly longer than the set between source (i.e. DAC) and pre where you’re driving a line level signal to the output. 

@ozzy I stand corrected. Somehow I thought if the preamp is in standby there will be current flow because some of the circuits are kept alive in the preamp but no. I just measured the interconnects with my integrated and there’s zero magnetic field when it’s in standby. You are correct sir! 

Explain please @ozzy 

if the cd player is playing it outputs signal thru the interconnects. If you don’t hear it because the preamp is off it doesn’t mean the signal isn’t flowing. 
If a tree falls in a forest and no one’s around to hear it, does it make a sound?

@wolf_garcia when I like the the sound of non broken in cables more, as soon as I think they broke in I reverse them (if it’s RCA) and run them for as many hours as it took to break them in. That restores the cables to the original state.
But this is a tedious process in your case with 400hrs on the Morrow. So every time you do that just flip them every few hours or so…don’t let them break in. 
It’s also very possible that you missed the magic moment the moment you bought the Morrow cables…