Paul - I have limited experience with XTOL. (And I can't speak to the Delta 400 question, although I know people who do get outstanding results with this combination.) I've only started using it recently, and so far only for Minox film. For my specific purpose - getter finer grain and softening the inherently high contrast in T-Max 100 - it seems to work quite well. (I have had to adjust Kodak's suggested development and agitation times for doing Minox film in the Minox Daylight Tank, but after a couple of experiments I'm getting very good results.)(posted 9124 days ago)But don't get swayed by XTOL's new-kid-on-the-block hype, which is so epidemic to the large component of photographers who are modern equipment and materials snobs: D-76 is STILL one of the all-time great developers! As ever, it represents nearly the best possible compromise between grain, sharpness, contrast, gradation, etc. For general purpose use, it's a wonderful developer. My father (a pro for over 50 years) still uses D-76 for everything that comes his way except sheet film (which goes into DK-50), and it works just fine, thank you.
With 35mm (or larger) Plus-X, Tri-X, T-Max, Agfapan, FP4, HP5, etc., it's an excellent choice. I've recently used it at 1:1 dilution with all those films, in 35mm (and some in Minox and 120 too) with very good results. Nice punchy negatives, a tad more grain that with stuff like Microdol-X (but MicX also gives you dull, flat negatives that require harder papers or VC filters to get equivalent prints), but with excellent sharpness, etc. And apart from blowing up the teeny Minox negs, that "tad more grain" is still nothing to worry about until you go above 8x10 with the old-tech 400-speed films, or 11x14 with the finer-grain films.
XTOL hasn't "replaced" D-76 any more than T-Max 100 "replaced" Plus-X and T-Max 400 "replaced" Tri-X. The older films and chemistry still produce magnificent results. This is not to say that XTOL isn't a fine product, it does seem to be a remarkable new developer with a lot of good points (normal temperture mixing, wide range of usable dilutions and temperatures, environmentally friendly, long keeping times after mixing, etc.) But, as far as I - and I suspect many other folks - am concerned, D-76 is STILL king!
After all, many (most?) of the greatest b/w photographs of all time were done decades ago on films like Panatomic-X, Super-XX, and Plus-X souped in D-76. It's really all about vision and skill, not equipment and materials... And the newer-is-better crowd have so bought into the wonders of modern technology that this little truth often gets lost in the shuffle. - MSG