The phono pre-amp need not be turned on. It's input impedence(thus a completed circuit) will remain constant, whether the unit is powered up or not.
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for peace of mind that you 'know' the breakin signal is 'getting thru'.....unplug the phono cable from the phono stage and plug it into a line level input on your preamp. verify the signal is getting thru and then you can turn down the volume if you like.
'proof' sometimes helps our mental comfort level.
don't turn on the phono stage with the breakin signal being run.; sending a line level signal thru the high gain MC input might not be good.
I'd be cautious about keeping the signal volume low, particularly if it is a solid state moving coil phono stage with a low input impedance. You would be subjecting the phono stage's input load resistor, and the junctions of the input stage active devices, to much larger voltages than they normally see.
I would guess that would be no problem, but without detailed design information (such as the power rating of the input resistors, and specifications on the ability of the active devices to handle input voltages outside of the zero-volt power supply rails they would have when turned off), I would not feel comfortable applying to them hundreds of times the voltages they were designed to handle.
Adding some to Al's concern of over driving the phono stage input...
If you want to be ultra safe, there is a Granite Audio Phono Burn-In & RIAA Test CD which I have used which would ensure protection. It provides for both MM and MC cartridge levels and includes RIAA compensation. You could drive it thru the tonearm wire and into the phono stage thus killing two birds with one stone.
Thanks to all for your comments and suggestions. A point of clarification: The tonearm wire is hardwired to the preamp's circuit board, and I am trying to avoid having to unsolder them.
Al, your concerns are valid, I will consider them. Zargon, while I understand the principle behind the burn-in CD (I have used, with success, the Cardas frequency sweep burn-in lp)isn't the idea to send through the wire a signal of much higher voltage than the tiny .2-4 mv normally outed by a cartridge, in order to burn it in?
Frogman, you are correct that a higher voltage may accelerate the burn in of the wires, however, as Al points out it may also cause harm to the phono stage assuming you don't unsolder the tonearm wires. My recommendation is to use the lower voltage produced by the CD and just run it longer. In truth, just playing records will also work just as well over the long run. It just depends on how anxious you are.