Hegel’s many references to Aristotle should help to clarify the Hegelian claim that “Substance is also Subject”. In particular, Aristotle’s own thesis of the identity of thought with the thing thought is relevant, as is his dialectical development of the different senses of ousia (“substance”) in the Metaphysics.
A thought for Aristotle is identical with its content. It just is a discursively articulable meaning, not a psychological event. What we care about in thought is shareable reasoning. Moreover, this shareable reasoning has a fundamentally ethical character.
Thought in this sense is essentially self-standing, and unlike the mental-act sense not dependent in the determination of its meaning on a “thinker” (who optionally instantiates it, and if so is responsible for the occurrence of a related event). This gives a nice double meaning to the autonomy of reason. (What such thoughts do depend on is other such thoughts with which they are inferentially connected.)
The primary locus of Aristotelian intellect is directly in shareable thoughts of this sort and their interconnection, rather than in a sentience that “has” them. Hegel adopts all of this.
Concepts in a unity of apperception are forms to be approached discursively, not mental representations or intentional acts. They are more like custom rules for material inference. The redoubling implied in apperception, like that of the Aristotelian “said of” relation, hints at the recursive structure of inferential articulation. The Hegelian Absolute, or “the” Concept, just nominalizes such an inferential coherence of concepts.
Thus, “Substance is also Subject” has nothing to do with attributing some kind of sentience to objects, or to the world. Rather, it is the claim that Substance properly understood (in the Aristotelian conceptual sense of “what it was to have been” a thing, rather than in the naive sense of a real-world object, or of a substrate of a real-world object, that Aristotle starts with but then discards) is already the right sort of thing to be able to play the functional role of a transcendental subject. A “Subject” for Hegel just is a concept or commitment, or a constellation of concepts and commitments. (See also Subject and Substance, Again; Substance and Subject.)
Consistent with this general approach, I consider the direct locus of the subject-function to be in things like Brandomian commitments and Kantian syntheses. The subject-function is also indirectly attributable to “self-conscious individuals” by metonymy or inheritance, and to empirical persons by a further metonymy or inheritance. (See also Subject; Substance; Aristotelian Dialectic; Brandom and Kant; Rational/Talking Animal; Second Nature.)