The answer most likely lies in curve shapes, or how the two films record the subject brightness range.(posted 8226 days ago)Unlike most "modern" films pioneered by Kodak with TMX and TMY, which have a straight-line curve shape way beyond the usable/printable range, new Delta 400 is a move back toward the "old-style" curve shape which has a bit of a shoulder.
Or to put it differently, Delta 400's highlights are reduced in contrast, or compressed.
Why?
The most common gripe about modern films is that highlights are too dense and unprintable. This isn't because of out-of-control processing; it's a function of curve shape. Ilford's first response was to produce a paper (MGIVRC) with an "abnormal" curve shape that served to compress the highlights in printing. Their second response was to produce films (D3200, new D400) with the "old-style" curve shape that could be more easily printed on "old-style" paper.
Combine a film that has lower-contrast highlights with a paper that also compresses highlights and you end up with grey muck. Try to print with more contrast (higher filter) and while you'll get more highlight contrast you may also get excessively-contrasty midtones and shadows.
So..the solution could be trying another paper. Agfa MCPRC has pretty hot highlight rendition, Ilford MGIV fiber and MGIVWT fiber are medium, and Ilford MGIVRC has reduced highlight contrast.
I'm not seeing a whole lot of change in D400 curve shape with various developers but otoh I've used only a couple of developers with it, which have been satisfactory (D-76H and Ilfosol-S).