Best DAC for my situation...Read on!

Are you comparing level matched to within 0.2 dB?

+1 for Benchmark

Your phone as a dB meter is not good enough, I am afraid. With such small but already important differences you really need to use a Volt meter. I appreciate that complicate things, but there is no alternative if you want a reliable answer.
There are three possibilities here, I think:
1 once the levels are exactly equalized the perceived difference disappears.
2 there is a gain mismatch between source and amplifier. This could be inside the amplifier, between the digital stage and the analoguie stage, or between the DAC and the analogue input. If the amplifier clips slightly, this may sound more lively and involving, even if in reality what you hear is distortion and compression.
3 one DAC is really better than the other. In this case, with two good ones, I doubt it.
Finally, why do you think that if the the inbuilt DACs of the Vault and the Oppo sound better than the DAC in the C48 you need yet another DAC?

The traditional purpose of a pre amplifier was to bring the different output levels of analogue sources to the same level (and add the massive RIAA equalization required by the phono input), to allow you to switch between these sources, to use tone and balance controls, and to control volume.
However, these days you do not necessarily need them anymore. Typical input sensitivity of a hifi power amplifier is now 2V, and typical cd red book standard output of a digital source is also 2V.
So what does one still need? Apart from tone and balance controls which you could also and better do with a mini DSP, you only need a volume control and a switch between inputs.
In a very basic system such as the system in my study, with just a computer plus ODAC usb DAC as a source, all I needed with my QUAD 405-2 power amplifier was a volume control like the Emotiva Control Freak.
With more digital sources like a basic streamer, a disc player, a tv, and perhaps a computer, one would need a DAC with multiple inputs and preferably a volume control. Examples of such units would be the Marantz HD DAC1, the Pioneer U-05, or various Benchmark DACs. Such DACs are in effect the modern incarnation of the traditional pre amplifier. Inserting a traditional pre amp after such DACs just adds distortion, noise etc. Pre amplifiers can only degrade the sound. And they are a waste of money.

If you are lucky (and you may well be if gain is correctly matched) you will hear no difference. If you hear a difference, you know the pre amplifier is the culprit.