Can I add a preamp of some kind.

I use Kodi and Raspberry Pi2 for music player exclusively. Works flawlessly. However, volume is too low on both Pis I use with stereos in different rooms. If I boost the rating up from 85db the sound becomes distorted.

Now, I don’t mind playing my stereo amps at 60 to 70 % power to fill the room, but is this necessary? I would rather get a preamp in the line somewhere between the Pi2 and the amp. Does anyone have a suggestion?
Thanks for any tips.