Balanced Streamer/DAC to Unbalanced Preamp

Nothing to be gained unless both are balanced, just use RCA

nope

As others have said, just use the RCA connection. If you ever get a pre with balanced input, you can try using balanced and see if it is an improvement to your ears. (Even if not, you might find, as I do, that XLR connectors are better designed and less likely to come unplugged or cause hum.)

No 

Yes

The only time I had to convert was when I was trying to use single-ended tubed preamps into a pair of large solid state monoblocks that only accepted balanced inputs.  The best way to do that was through a pair of transformers such as the Jensen ISO-MAX products or  those made by Lundahl.  I used Jensen, but with either you need to check the impedance matching since the transformers are sensitive to a narrower range of input/output impedances before rolling off frequencies.

As the others said, just use the RCA connections.

Thanks all. It's what I thought but great to be able to get some quick advice here!

Nope!

Check the specs on the Lumin D2 and make sure the output impedance is the same from the XLR and RCA outs. If there is a difference then that could mean using the RCA outs limit the DAC’s potential.