What you describe is possible. By removing the I/C's from the preamp, the shielding path is broken and the I/C's pick up external RF and EM fields. If there's a loud hum, the amplifier is probably leaking a small amount of current to ground. You can check this by unplugging the I/C's to the amplifer, and see if there's humming at the speakers. If the hum is almost as loud or very noticeable, check to see that the input tubes and/or rectifer tubes are good. If the hum is only noticeable with your ear close to the speaker, then the amp is okay.
Next, connect the I/C's to the amp and preamp. If the hum is louder, then you have a ground loop problem. In addition to the suggestions above, try plugging the amps and source equipment into just one of your power conditioners and see if the hum decreases. The way you're set up now is ideal for ground loops - especially with long, unbalanced I/C's.
BTW: please make sure the amp and source equipment are off before you unplug or plug anything. Good luck and happy hunting - unfortunately, that's what tracking down a ground loop is.
Next, connect the I/C's to the amp and preamp. If the hum is louder, then you have a ground loop problem. In addition to the suggestions above, try plugging the amps and source equipment into just one of your power conditioners and see if the hum decreases. The way you're set up now is ideal for ground loops - especially with long, unbalanced I/C's.
BTW: please make sure the amp and source equipment are off before you unplug or plug anything. Good luck and happy hunting - unfortunately, that's what tracking down a ground loop is.