As a previous owner of R8, I would be happy with both and not miss the other - in terms of sound. There are some differences in sound but if I am not doing a side by side comparison and aiming just to detect that - at least with my transducers - there wouldn't be a clear winner for me as both have different points to like about. Other than that, it comes down to other features.
- R8 has a real and separate LO, DX300 (at the moment) does not. It is a useful thing to have them separated if you are using the LO often and want to reduce the risk of accidentally forgetting to switch to PO mode.
- DX300 is lighter, R8 is quite heavy.
- R8 is shorter but DX300 is as long as a mobile phone (could be positive or negative).
- But in hand, DX300 is easier to control - especially the volume, which is a downside of R8 design (at least for me).
- R8 has a larger battery and does not have the charging quirks of DX300. Whereas DX300 has a more energy efficient DAC setup, so spends less energy there, ending up with about the same play time and less heat.