With Hexplosives Expert and Hunters' Circle, is the bomb's cost reduced by an energy when played to that battlefield?

No — the Bomb's cost is 0, so Hunters' Circle's "[1] less" has nothing to reduce, and Hexplosives Expert's activated ability still costs the full [1][A].

The [1][A] is the ability's cost (the part before the colon), not the cost of the gear you're playing; Hunters' Circle discounts what a unit or gear costs to play there, and tokens don't have costs — they're treated as 0 for all purposes (Rule 185.3.a.1). A discount can't reduce a component below 0 (Rule 356.4.c.2), so nothing changes. Deploy does let you put the Bomb at Hunters' Circle (Rule 830.1.b.1), so the play location is legal — but note it still counts as the first gear you played there that turn, so you'd burn Hunters' Circle's once-per-turn discount on a 0-cost token.