The only alternative is to get some type of HT processor.
The idea that “if inserting a pre amplifier changes the sound, the pre amp is at fault”. This is totally incorrect and goes against all audiophile ideals, it is based on on ideal that if a piece of equipment is good than it will sound no different from any other piece. The truth is that almost every audio device out there sounds slightly different. One of the purposes of a preamp is to help “shape the sound”. It is a tool that can be used by an audiophile to help achieve their goals for the type of sound they want.
I have have heard the Oppo 105 straight to an amp. It sounds good, but it can be slightly dry. A tube or Class A preamp can help add richness and engaging sound.