I've been using Magix music production daws since 2007 and have recently upgraded to Samp Pro X 5. I've always had issue with overly loud soundpool loop playback prior to inserting loops onto to tracks. I've done a brief search on forums to see if i am missing something in controls but it seems i am not. I did see someone post a comment about this very subject and someone did answer saying that in the playback dock, music maker edition, the loop playback had its own volume adjust. I would expect this to be included in any Pro version as standard.
The soundpool (pre-listen) is really extremely loud and is a big turn off when trying to select loops/samples. This happens even when turning down main volume in software mixer or my hardware mixer to compensate. There is no way around this. The samples/loops are quite distorted in comparison and if the user is not aware, this could be potentially damaging to the listeners ears. I use a mix of sample/loops and live instruments and vocals.
I have searched extensively in SPX5 for the feature but it is not there. Therefore, please look into this as a future bug fix/addition or update. It is very important.