Component of 1,2-phenylacetyl-CoA epoxidase multicomponent enzyme system which catalyzes the reduction of phenylacetyl-CoA (PA-CoA) to form 1,2-epoxyphenylacetyl-CoA. The subunit B may play a regulatory role or be directly involved in electron transport. Aromatic compound metabolism; phenylacetate degradation. Homotrimer. Forms a stable heterodimer with PaaC. Probably forms an oligomer with PaaAC. Activated by cAMP receptor protein (CRP), integration host factor (IHF) and by phenylacetyl-coenzyme A (PA-CoA) that prevents PaaX from binding its target sequences. Inhibited by PaaX.