When choosing a Bluetooth format for the iFi Audio Zen Air Blue, several factors should be considered, especially since you are connecting it to an old but good receiver via RCA (cinch) cables.

### Key Considerations
- Sound Quality: If you prioritize high-resolution audio, you should opt for formats that support higher bitrates and better sound quality.
- Device Compatibility: Ensure the format is supported by both your source device (e.g., smartphone, tablet) and the Zen Air Blue.

### Recommended Formats
- LDAC: This format is highly recommended for its ability to transmit high-resolution audio at up to 24-bit/96kHz. It is supported by the Zen Air Blue and offers excellent sound quality.
- aptX HD: This format also supports high-resolution audio up to 24-bit/48kHz and is a good alternative if your source device does not support LDAC.

### Other Formats
- aptX Adaptive: This is a versatile format that adjusts bitrate based on the connection quality, ensuring a stable and high-quality audio stream. However, it may not always reach the same high resolution as LDAC or aptX HD.
- AAC, SBC, aptX: These formats are more general and may not offer the same level of sound quality as LDAC or aptX HD, but they are widely supported and can be used if the other formats are not available.

### Conclusion
For the best sound quality when connecting to your old receiver via RCA cables, LDAC or aptX HD would be the optimal choices, provided your source device supports these formats. If not, aptX Adaptive is a good fallback option for a balance between sound quality and stability.