Andrew: Thanks for the suggestion - I can get to _some_ concentration between 1+4 and 1+9, and then go with the clearing time test again.
Gerald: I didn't follow the mixing recommendations on the concentrate bottle after I messed it up - so what I have right now isn't a "more dilute" version of what I ought to have had.
I compared the negatives with my earlier samples, and the latest ones definitely have a purple cast to them.
Just eyeballing the latest negatives with earlier "known good" versions, it looks like the overall density is pretty close, but the negatives are less contrasty than what I was used to. Reviewing my notes, it looks like I made one further mistake. Normally I would start my solutions at 70 degrees F, while using the dev times for 68 degrees (the Ilford doc on processing suggests this since the temperature would fall slightly during development - I am using plastic tanks, so I can't control the temp. once the solutions are poured in). But the latest set of negatives was done starting at 68 degrees. Maybe that is leading to slight underdevelopment, and hence the loss in contrast?