Honashagen,
Your question is essentially that you are asking what level of quality you need to attain to improve on Oppo 105 direct to your DNA amp. (Or possibly how to find it, but the answer to that is obvious, trial and error, or, asking an expert with more experience, hence, presumably your post.)
The discussion up to now seems to me to be ignoring one obvious aspect. That is, what you are really trying to do is improve on the built in preamplifier that comes with your OPPO 105. To really and truly bypass your Oppo 105, you’d pretty much have to take a digital out signal and go with a preamp that takes the digital signal and modulates that into the analog domain. Otherwise you are using some of the preamp circuitry of the OPPO already.
I believe (and I may be wrong) that your Oppo uses different pathways for its RCA outputs whether line level or variable out, than it does for its XLR outputs.
It means you have potentially two different quality preamps built into the Oppo, and you’re listening to just one of them.
Consider a preamp might outperform one, but not the other.
For example if if you had a Bryston amplifier with both RCA and XLR, you might prefer one to the other, and require a preamp taking in the digital signal to improve the one you don’t like to get to the level of the other.
Before I’d blindly assume a preamp which is just as likely to color poorly as well as improve it, I’d also equally consider an amp with true balanced technology to see if the OPPO does a better job via its XLR outputs. (ie you may already own a better preamp, you are just not using it)
I guess I’m just pointing out you already have a preamp in the OPPO.
