The Samsung HW-Q930D and HW-Q930C have some similar features, but there are some differences and nuances that should be considered:

## Main differences between HW-Q930D and HW-Q930C
- HDMI version: There is no official confirmation that the HW-Q930D supports HDMI 2.1. Both models seem to offer 4K@60Hz pass-through only.
- Firmware and software: The HW-Q930D may have minor firmware updates or software improvements that are not present in the HW-Q930C. However, these changes are rather marginal and do not affect the basic audio performance.
- Newer model range: The HW-Q930D can be considered the newer variant of the HW-Q930C, although the differences are very minor.
- Additional features: There is evidence of a new "Party Mode" in the HW-Q930D, where each pair of speakers can perform the same audio playback separately, which is not available in the HW-Q930C.

## Comparison with the HW-990C
- Channel configuration: The HW-990C has an 11.1.4 channel configuration as opposed to the 9.1.4 channel configuration of the HW-Q930D. This can result in a richer and more immersive audio experience.
- Maximum volume: The HW-990C offers approximately 20% higher maximum volume than the HW-Q930C/D.
- Other features: The HW-990C may have additional features such as improved HDMI 2.1 support (especially for higher frame rates) and further enhancements to Q-Symphony technology that are not present in the HW-Q930D.

### Recommendation
If you are already happy with the HW-Q930C and don't need the specific additional features of the HW-Q930D or HW-990C, the HW-Q930C could be a good choice due to its lower price. However, if you are looking for a more comprehensive audio experience and are willing to invest more, the HW-990C might be the better choice. The HW-Q930D offers essentially the same audio performance as the HW-Q930C, with only minimal improvements.