Oppo 105 in place of pre/pro

Certainly, you can. It has all the facilities for a very basic HT system. However, you should read the user manual for the specifics of that application: http://download.oppodigital.com/BDP105/BDP-105_USER_MANUAL_English_v1.8.pdf

Would the 103 also work for this?

I tried using my 103. The sound was ok however, the added gain and better switching from a pre amp helped a lot. I still use the analogue outputs from the 103 and video through HDMI. The internal source switching on the Oppo is not as easy as with my Emotiva. Also, the volume on the Emotiva is much more responsive and seems to sound better than the digital volume from the Oppo.
Try it and see for yourself...

Right. I was not recommending this use, merely saying that it was possible. Its major advantage is cost and that applies to 103 and 105 (and 103D and 105D).

Send your request to Oppo contact on there site, they have extremely good service. They send me some answered that I had, the same day and it was Christmas day….