I have had the 5se and now the ref 6. Both are dead quiet with and without stock power cord. I believe all of your tests you will do will be of no help. Tubes , power supply , power cord etc. I believe something happened in transit. Especially that the tubes were left in the sockets. That should of definitely not have been shipping like that. I also feel that there is absolutely no reason for u to live with your 5 se making a humming noise. I definitely would not live with that no matter how minor it is. Or that it only reappear in between tracks. There is absolutely no reason for that. Send it in to Arc for repair and get it back to normal operating condition. I would also contact the seller and tell him that he didn’t ship it the right way and see if u can work out something that it has to go into service for his negligence. Good luck.