I would put my preference in terms of connection methods as:
1. USB (Particularly for multichannel audio)
2. Digital Optical or Coaxial (Only supports stereo)
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5. HDMI (Can have connection issues, and hence multichannel audio issues)
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10. DLNA/Ethernet (Totally dependent on the DLNA Renderer, and can be unreliable or have limited capabilities. Can also work very well.)
Maybe I am being harsh. But how well DLNA works is entirely dependent on the capabilities of the DLNA Renderer. Even high end and very capable audio devices may have quite poor DLNA Renderer implementations, that are significantly less capable than direct cable connection methods.
The position bar should work fine when using MC to control a DLNA Renderer. As above, it depends on the Renderer capabilities. MC has some setting to get around Renderer shortcomings. Finding the correct settings can take a while, and may require analysis using AndrewFG's Digital Media Renderer Analyser (DRMA).
Correct, "Locate on disk" only works when Windows (or Mac OSX I guess) sees the storage device as a drive. That could be a local drive, a USB drive, or a NAS using mapped drive or URIs. It doesn't work if MC is connected to another server which is serving the files, such as a MC Server, or your Minimserver.
If the sound is different over ethernet using DLNA, compared to a USB connection, and the exact same device is being used, then it is likely that there are settings differences. Or simply a volume difference. DLNA is a file-based solution. It sends the file to the DLNA Renderer which then plays it. As such, it is basically the same as playing a file via USB to the same device. But the DLNA Renderer may "color" the sound, or in the case of your Marantz SR6013, may process the audio differently. Of course, if you are comparing a USB DAC to the Marantz SR6013 connected via DLNA, then you are likely to hear differences just due to using different devices.
But your Marantz SR6013 doesn't have a USB input, except on the front panel and that seems limited. Correct?