Have you read the post by Gudzinowicz in re.photo.darkroom? It is quite enlightening.
Test the water solubility of your p-aminophenol. If it is in fact the base, it should not be very soluble. The hydrochloride is more soluble than Metol.
I don't think that is the problem, though. Of course, our pH meters may not be calibrated alike, but the pH of my final solution with just a little precipitate reads less than 12.
Traditionally, the preparation of Rodinal begins with p-aminophenol.HCl in solution. Adding potassium metabisulfite does not cause it to precipitate. The first addition of hydroxide, either Na or K, removes HCl from the tail, which makes the flocculent precipitate. As more hydroxide is added, the potassium or sodium aminophenolate salt is formed, which is the final developing agent, and is quite soluble in water. If you have pure ingredients, the amount of each one required to convert all the p-aminophenol.HCl to a solution of the aminophenolate salt and NaCl or KCl can be calculated. The shelf life of the Rodinal stock is much extended if some of the p-aminophenol base is left undissolved. I do not know why, but apparently Gudzinowicz does. The amount of hydroxide required to remove the HCl from the hydrochloride is, in molecular weights, the same as the amount required to make the aminophenolate, so using the p-aminophenol base in place of the hydrochloride will require only half as much hydroxide. Last time I bought any, a pound of the base cost the same as a pound of the hydrochloride, so I saved money two ways.
Obviously, impurities anywhere along the line can bolix up the calculations. The impurities are not likely to affect the performance of the developer if the traditional titration is done at the end. You will wind up with a solution of sodium or potassium paraminophenolate and a sulfite in water with some precipitate which should be kept with the stock. You may not know the exact concentration of the stock if part of the p-aminophenol was the base and part was the hydrochloride, but calculating the concentration from the nominal weight of p-aminophenol and the final volume will be close enough for government work.
What bothers me most is the difference in pH between yours and mine. That could, of course, be due to differences in calibration between meters, were it not for the test strips.