The eARC function of the Apple TV box is for using the Apple TV box as audio processor for TV content when using HomePod speakers. It doesn’t apply to AV receiver use.
For use with AV receivers, only the eARC function of the TV applies.
It is unclear what you mean by “without TV”. eARC is all about the use with a TV. The Apple TV box requires a screen, which usually is a TV.
The ARC/eARC connection is only used between the screen (TV) and the audio processing unit (often: AV receiver). It is not used (doesn’t apply) for the connection between the Apple TV box and the AV receiver.
- Apple TV box →[HDMI]→ AV receiver →[HDMI ARC/eARC]→ TV, or
- Apple TV box →[HDMI]→ TV →[HDMI ARC/eARC]→ AV receiver
The first option requires that the receiver supports passthrough of the video signal in all its glory. That may be an issue for older or not-so-high-end models.
The second option requires that the TV supports passthrough of the audio signal in all its glory. That may be an issue for older or not-so-high-end models.
Please specify the model receiver and model TV for more focused advise, so that we may have the possibility to look up technical specifications.