Jamieson, Fausset and Brown on Hebrews 1:7

7. of--The Greek is rather, "In reference TO the angels."

spirits--or "winds": Who employeth His angels as the winds, His ministers as the lightnings; or, He maketh His angelic ministers the directing powers of winds and flames, when these latter are required to perform His will. "Commissions them to assume the agency or form of flames for His purposes" [Alford]. English Version, "maketh His angels spirits," means, He maketh them of a subtle, incorporeal nature, swift as the wind. So Psa 18:10, "a cherub ... the wings of the wind." He 1:14, "ministering spirits," favors English Version here. As "spirits" implies the wind-like velocity and subtle nature of the cherubim, so "flame of fire" expresses the burning devotion and intense all-consuming zeal of the adoring seraphim (meaning "burning), Is 6:1. The translation, "maketh winds His messengers, and a flame of fire His ministers (!)," is plainly wrong. In the Psa 104:3, 4, the subject in each clause comes first, and the attribute predicated of it second; so the Greek article here marks "angels" and "ministers" as the subjects, and "winds" and "flame of fire," predicates, Schemoth Rabba says, "God is called God of Zebaoth (the heavenly hosts), because He does what He pleases with His angels. When He pleases, He makes them to sit (Jud 6:11); at other times to stand (Is 6:2); at times to resemble women (Zec 5:9); at other times to resemble men (Ge 18:2); at times He makes them 'spirits'; at times, fire." "Maketh" implies that, however exalted, they are but creatures, whereas the Son is the Creator (He 1:10): not begotten from everlasting, nor to be worshipped, as the Son (Re 14:7; 22:8, 9).