Catalyzes the conversion of (2E,6E)-farnesyl diphosphate (FPP) into (+)-corvol ether A and (+)-corvol ether B via a 1,10-cyclization, which requires isomerization of FPP to nerolidyl diphosphate (NPP) and then abstraction of the pyrophosphate from intermediate NPP leading to a (E,Z)-germacradienyl (helminthogermacradienyl) cation (PubMed:25809275, PubMed:27666571). The preferred substrate is (2E,6E)-farnesyl diphosphate (FPP), however geranyl diphosphate (GPP) is also able to produce small amounts of several acyclic and cyclic monoterpenes, with linalool as the main product (PubMed:25809275). (2E,6E)-farnesyl diphosphate + H2O = (+)-corvol ether B + diphosphate (2E,6E)-farnesyl diphosphate + H2O = (+)-corvol ether A + diphosphate Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. The Asp-Asp-Xaa-Xaa-Xaa-Asp (DDXXXD) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family.