I have not seen the schematics for the 990. Normally a balanced output with the unbalanced option take the + driver to the unbalanced RCA.

While you can connect both balanced and unbalanced under this scenario, you negate some of the benefits of balanced audio, causing a mismatch between the + and - legs of the balanced connection. This will reduce ur Common Mode Rejection or imunity to noise which is the entire reason for balanced connectors in the first place.

Thanks,

Eric