Actually, since demon lords advance by +1 CR for every Hit Die you add, their spell resistance does actually increase by 1 for every HD gained. In my turnover, all the demon lords had a base SR of CR +13; I really don't know why this was changed during the book's editing/development cycle. Anyway, as a result, when you advance several demon lords, they'll get a 2 point boost in their SR. <shrug>
As for the Dragon issues:
Dragon 329: Pazuzu (plus anzu demon)
Dragon 333: Fraz-Urb'luu (plus skurchur demon)
Dragon 337: Zuggtmoy (plus basidirond, phycomid, and vathugu demon)
Dragon 341: Baphoment (plus ankishar demon)
Dragon 345: Kostchtchie (plus mavawhan demon)