I encountered this too in the beginning. I simply increased the channel gain in Cubase to +6dB.
When adding more instruments the mix is getting much louder, so I ended up decreasing the channel volume to 0dB later on again.
So I guess the Mixing Presets are actually balanced to be used in an arangement. However, I found in general that the synchronized Samples are much better pre-mixed than the Synchron ones. Out-of-the-box you don't need to tweak anything with r.g. the special edition, but combining Synchron Brass and Synchron Strings, I found that you have to adjust the sections and also individual levels alot.
Also, the dynamic range is pretty big in Synchron. So you could try to reduce it externally (compressor) or internally with the CC (there is an option).