Usually I enjoy the use of the main theme as a final boss battle BGM, but there are exceptions. Unleashed is definitely one of them, I hate its main theme. The actual final boss theme used is decent, not the most epic, but it suffices.
The other is Sonic Heroes, the title theme works as a title theme, but for battle, What I'm Made Of was infinitely better (WHY they didn't use it instead of the Sonic Heroes theme for Brawl, I will never know).
The game that definitely SHOULDN'T have replaced the title theme is Adventure, for Perfect Chaos part 2. The spontaneous switch really breaks the flow, and the second BGM blows anyway.
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