Thanks for your suggestions!
The problem is that as far as I can make out, no matte paper has a decent Dmax or is much good in the reds/oranges/violets. Here for example is the Hahnemuhle William Turner (4880 profile from Epson):
Other papers like the Epson Radiant White (seems to be called 'Exhibition Watercolor Paper Textured' now) are slightly better in some areas and slightly worse in others, but all these papers have a Dmax of under 1.6 ... which is probably par for the course for matte papers.
I can live with that, sort of, but the weakness in the reds/purples is really a problem. In comparison, a paper like the Canson Platine Fibre Rag (which is not a matte paper) has a Dmax of nearly 2.2 and covers most of sRGB and a good bit a AdobeRGB (and exceeds both in areas).
Alan, many thanks for offering some sheets of the William Turner ... but I'm in Ireland (and I guess you're in the U.S).
What I'm doing at the moment is printing the less saturated images on Hahnemuhle Photo Rag and Epson Watercolor Radiant White and the more saturated ones on Canson Platine.
If anyone is interested in trying out a problematic image on some watercolor paper, you can download it here:
http://www.irelandupclose.com/customer/LL/Poppies#4.jpgWhere there's a real flattening is in the darker reds on the large flower: the whole flower ends up looking quite flat using a matte paper:
Printed on Canson Platine it looks great ... but then it looks more like a print of a watercolor than a real watercolor.
Cheers,
Robert