I’m don’t have experience with tubes, but if it’s one thing I have learned through a lot of R&D is that if the sound signature/quality is not there from the source, you are not going to get it back with any preamp magic. Not to say that the Oppo 105 sonics are bad -- they really aren’t, but adding a tube preamp is going to layer a tube signature on top of a solid state signature. Don’t get me wrong, this may work very well in a certain combination of equipment.
When you use the digital volume in the Oppo, the Oppo will convert the 16/24 bit PCM up to 32-bit PCM and then trim bits to reduce the volume. This supposedly works well, but I can’t keep thinking that something is lost because the DAC has to do processing on 32 different ladder voltages. I don't know how Sabre does this, probably by re-using ladder pathways in a circular pattern (I definitely don't think it actually has 32 ladder resistors, probably more like 11 if I remember right).
You could try doing the Modwright Oppo tube modification and then use something like a Khozmo stepped attenuator passive preamp for volume. I’d ask Modwright about the feasibility of this and which impedance value he recommends (if any) for the passive preamp.
Just an idea.