Okay Ben...
Seems we've eliminated lots of the usual suspects, so let's look for some not-so-usual ones
Since the emulsion doesn't seem to be affected, I'm thinking that the stain is in the paper base or maybe even in the baryta layer (this latter would be unlikely and definitely a paper defect.
So (just brainstorming):
Can you see the stains on the back of the print? Or do you need through-lighting to see it?
If the stain is easily visible on the back, then...
Are you making sure that the backs of your prints receive as much wash time as the fronts?
Is there any way that the back of the print is getting contaminated?
Could you be getting staining from your developer somehow? (see below).
If you need through-lighting to see the stain, i.e., it's not visible under normal surface lighting from the back or the front, then that means it is "inside" the paper somewhere, e.g., under the emulsion but somehow not on the back surface. If that's the case, try Michael's suggestion of toning unexposed prints, one that's just been fixed and one that's been developed and fixed. If the stains only show up on the developed one, then there's one set of possibilities, if it shows up on the one that was only fixed, then another.
Staining on the developed one would point to some kind of fogging or staining from the developer. Staining on a print that's only been fixed... well, I'd be contacting Ilford about that.
Whatever is going on, it's not normal and somehow, with a different paper batch or whatever, you'll be able to resolve it. So don't give up hope!
FWIW, I've been using more MG Classic these days and have had no such problems. However, I did have some problems with Adox MC-110 and Liquidol together. Somehow the developer stained the emulsion. Something similar might be going on in your case. In my case, however, the staining was visible right away, i.e., when I turned on the light over the fixer tray.
One more question: Is the staining just along the cut edge of the paper? In other words, is it just where solutions could seep into the interior of the paper from the edge. If so, I'd be contacting Ilford and printing with extra-wide borders so I could trim them off later
Keep us posted.
Doremus
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