Bonjour le suédois, įa va?
Strange problem indeed ... and we should check that you *really* have a problem.
1) you say that 2 meters or more away, you have no hum problems, with none of them ?
2) you say, that, say, at 1 meter, you have "enormous hum" with the 3rd one, but not with the other two ?
Iīd recheck this, with all three, one by one, with the amp in the exact same place on the bench, the settings in the exact same position, the bass in the exact same place too.
You may stick some pieces of tape on the bench to mark the positions of head and bass, so you repeat the experiment with the three of them, under the exact same conditions.
Otherwise you are comparing apples to oranges.
Iīve chased gremlins/ghosts many times before trying to solve "problems" which really werenīt so, simply different sttings (which went unnoticed) or which later couldnīt be reproduced.
Why do I insist on experimenting in such a "rigid" way?
Because in fact, it is *normal* for a Bass (or Guitar) pickup to pick up transformer hum at a short distance, and for transformers to have stray field .
What I donīt get is one having *lots* more than two exact "brothers".
The only condition I imagine causing that would be the culprit having a shorted turn, or trying to power a shorted PSU or power board , real gross failures which would blow fuses or cause heavy overheating in a short time; you report none of that.
PS: *unless* two of them have, say, toroidal transformers and the suspect one, being from another year, has a conventional EI transformer, but I doubt it, just mention it as *something* that could explain two similar VA rated transformers having a very different stray field.
PS2: *maybe* the culprit is unstable or oscillating, (wouldnīt surprise me at all) placing a heavy load on the PSU (and the transformer) and transmitting all sorts of interference.
Just in case, scope the output when that happens.
Maybe *you* hear it as "hum" with human ears, but strange RF things are happening ... which you donīt directly hear, only its artifacts.