Faulty tubes: Even though you have installed new tubes, it's possible that one of them could be defective or not seated properly. Try reseating the tubes to make sure they are making good contact with their sockets. If that doesn't fix the issue, you may need to try swapping the tubes out one by one to see if you can isolate a bad one.
Power supply issues: A loud hum can sometimes be caused by a problem with the power supply of the amp. This could include issues with the power transformer, filter capacitors, or other components in the power supply circuit. If you are not experienced with working on tube amplifiers, it's best to take it to a qualified technician to have the power supply checked out and repaired if necessary.
Grounding issues: Improper grounding can also cause a loud hum in a tube amp. Make sure that the amp is properly grounded and that there are no loose or broken ground wires. You should also check that the power outlet you are using is properly grounded.
Damaged components: If the amp has been subjected to physical damage or excessive wear and tear, it's possible that some components could be damaged, causing the hum. This could include issues with the input jack, output transformer, or other components in the signal path. If you are not experienced in diagnosing and repairing tube amplifiers, it's best to take it to a qualified technician for further inspection and repair.