Source Mod or Preamp?

I think you are safer with a tubed preamp. I have a lot of experience with tubes and they are very good at adding dimension to your sound.  They will be very clean sounding and  not that's right not   syrupy slow and warm, using current production tubes.  If you want to add a hint of warmth there are some tubes that may help with that as well. 

Modded anything is harder to sell since there are fewer people familiar with the mods and are wanting to buy a specific component.  So if fluidity is important to you extensive mods are not a good choice.  The cost will likely exceed some of the less expensive tubed pre amp units.

ARC has a particular presentation that I think you should be familiar with before buying if you don't know how they sound already.   I own both an old ARC SP6B (modded) and a tubed pre, a simple AE-3 (an offshoot of Cary).  I love that little pre.

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.