In addition to echoing what others have said, I'd just like to point out that Diafine works and plays *really* well with HP5 (EI 800 in my experience). Lots of detail pops out of the shadows, and the shoulder produces nice natural-looking highlights. It works really well for street shooting at night.
Tri-X in Diafine has a look all its own---one of those "when it's what you want, it's what you want" things. The instructions say EI 1600, but a lot of people prefer it at 1250. You wouldn't think 1/3 of a stop would make that much difference, but it does.