John, If you develope your film with a large tank, filling it partially, you will run the risk of underdevelopment. Such treatment will oxidize the developer much more rapidly, which may increase staining, but will also significantly diminish developer activity. It will also make it difficult to achieve consistency. For example, how would you determine a time if you develop two rolls in a 1 liter tank (using 500ml of solution) if you established a time for one roll in the same tank using 250ml? This problem, specifically, is why Rollo Pyro was developed. In tubes or Jobo-type roller processing, the developer oxidizes too quickly, causing underdevelopment. Hutchings suggests adding 30% more "A" solution to counteract the oxidation. Rollo pyro adds vit-C and much more pyrogallo to solution A. Rollo Pyro is another option you can try, but I think you can anticipate considerably more grain with 35mm film.(posted 8389 days ago)Personally, if you aren't getting adequate stain, I'd try the suggestion of letting the developer sit for an hour before use, or try Hutchings' suggestion of increasing the A concentration.
As for capacity, Hutchings states that 250ml is adequate for one roll of 35mm. Anchell/Troop were speaking in general terms about all highly dilute developers. I think 250ml is adequate with PMK, but, with that said, I always fill a 500ml tank for one roll and 1000ml for 2 or 3 rolls.
If all else fails, you can mix up a solution of sodium carbonate, using a tablespoon or two (precision is not important with this) in one liter of water. Then, instead of redeveloping in the used developer, treat the film in the carbonate solution as you would with used developer. This will increase stain considerably. However, it will not be as descriminating as used developer--it will add stain to the entire film, not just the image area. This works quite well with films like TMX that resist staining.
I can't understand why you're not getting adequate stain with HP5+. Fast films stain better than slow, and HP5+ should stain very well. Why don't you try mixing up a batch of TF-3 fixer (Film Developing Cookbook/Anchell&Troop--if you don't have this book, get it; you'll love it!) ammonium thiosulfate (57-60%) 800ml, 60g sodium sulfite, 5g sodium metaborate, plus water to make 1000ml. Working dilution is 1:4 with water. I recommend distilled water. You can also use TF-2: 750ml water, 250g sodium thiosulfate, 15g sodium sulfite anhydros, 10g sodium metaborate, water to make 1000ml. Use undiluted. I prefer the TF-3, but TF-2 will at least give you a basis for comparison with your Agfa fixer. Good luck!