I would use the Record Outs those should be a set line level and your HP amp will do the amplfication for volume.
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If you must leave the cables connected, get a pair of cardas caps to put on the ends. They can create noise, etc. Basically act like little antennaes.
Keep the volume low on your preamp and see what happens when rec/out is engaged and the cables are not connected (it will undoubtedly happen accidently sometime in the future so best to know now, at a low volume level). On my preamp, I get a vicious shriek, but I do have a rec/in connected as well.