Extra Stout.

[Jrrj]

For Foreign Extra Stout I've used wyeast 1028 per Ray Daniels' guidance in Designing Great Beers. Excellent results. Daniels recommends London over Irish stout strains from memory due to attenuation not being high enough. From 1.070 London should get you down into the high teens similiarly to what you reported for 1728. London is also very fast - reaching terminal gravity inside 4 days at 20 degrees in my experience.

 

[GalBrew]

If you want to stick with a dry yeast I have found that s-04 works really well with dark ales. Just make sure your pitching rate is adequate. I usually pitch 2 rehydrated packs into a stout of that OG.