I think starting out using the Oppo as a pre is good. But does not keep up with a good pre just does not have enough voltage to drive a amp at it best capability . I have a cat SL1 ultimate. And a Casse Omega pre they both smoke the Oppo as a pre. And they should they are 10 times the cost. Even my old classe CP50 smokes the Oppo as a pre. You can get a CP50 and CP60 used for very little money if you can find one. They are true balanced from input to output. And fabulous sounding preamps. Also Audible Elusions (SP) are great preamps as well. Again trust your EARS ONLY! This just my opinion.
FYI the signal that comes out of the OPPO 105D is just as clean and good as that of any preamplifier.
There is almost NO difference in putting out a standard RCA output to a preamp vs. driving an amplifier in terms of electronics needed and in the case of the OPPO 105D it has a fixed amplification going out and just adjusts the signal with the DAC.
HOWEVER how well it drives an amplifier is very dependent on the input impedance of your amplifier.
Many Pre-amps and amps are matched and an amplifier can have an input impedance from around 20 kohm up to to 2 Mohm.
For example the Wyred4Sound have a 100kohm and 96kohm input impedance.
A Bryston has a 20k ohm balanced and 50k ohm single ended.
My amplifier has a 33 kohms balanced input impedance and the OPPO works very well with it. OPPO suggests a perfect match as a 47kohm amplifier input impedance.
I tried a Bryston BP6 which is about a $3,000 preamp and it did no better than my OPPO 105D did.
The OPPO 105D will
NOT drive all amplifiers well. That is very true, but if you have an input impedance that matches well it does a great job.
That said ideally you have a matched amp/preamp