To increase stain, Hutchings recommends mixing PMK an hour or so before use. The color of the developer gets darker as it sits, which I presume is oxidation. I've had a little luck with that trick. And I think I'll try the bigger tank idea. 250ml of developer with one 36 exposure 35mm film seems to work just fine at my house.(posted 8493 days ago)To increase speed with HP5, Hutchings recommends double-strength PMK. I think I've gained maybe half a stop that way. It also seems to help P3200.
The only way I think of that the Agfa fixer might be hurting stain is if it is high in sulfite. A lot of commercial preparations are. And you are NOT using an acid stop bath, right? (Only water.)