Heb 1:3 is a very important verse, one that shows that God the Father has a body just as the exalted Jesus Christ (see discussion here). I encountered the following from a Reformed author today. While the author does not believe that the Father is embodied as the Son is, the logical conclusions of his comments would fit the traditional Latter-day Saint interpretation of this verse if he were consistent (and were to back up the [false] claim that απαυγασμα is in the active voice [discussed in the exegesis provided above]):
[A]s ‘the radiance of God’s glory’, the Son is the perfect manifestation of God’s glorious presence. In the second declaration the term rendered ‘exact representation’ (charakér), which was used of a mark or impression placed on an object (e.g. of coins), signifies a ‘representation’ or ‘reproduction’ of the ‘being’ (hypostasis) of God. The Son is the exact representation, the perfect embodiment, of God, as he really is. To see him is to see what God is like. The Son is uniquely qualified to be God’s historical self-revelation because he is identified with Yahweh himself. (Peter T. O’Brien, God Has Spoken in His Son: A biblical theology of Hebrews [Downers Grove, Illin.: Intervarsity Press , 2016], 49-50)