I do have the Marantz equipment available, but the swap would take time. i'll keep at it. Thanks!
Just thought I would share this in case you have not done this before!
I would approach this as follows:
1) list the specific instances where there is a difference (you have already done that in your post above)
2) Get one of these for $10 to quickly switch between preamps:
Buy AuviPal 4-Way RCA Switcher 4 in 1 Out Composite Video L/R Stereo Audio AV Selector Box for DVD VCR VHS/AV Receiver/ PS2/ Nintendo Game Consoles: Selector Switch Boxes - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
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[I assume the differences show up when listening in stereo. The reason I would chose a 3 way or 4 way switcher (even though you are only testing two way) is because I don't trust these switches and am not confident that sending two signals into one channel of an amp is safe, but if you select a "null" channel between active channels, there is no chance of accidentally having dual signals sent to the amp at the same time].
3) I assume you have two live outputs from whatever source you use (for example, my BD Player has HDMI and TosLink, so I can feed one to one pre and one to the other). This way you don't have to switch anything.
4) Start out with both pre's in pure direct - Level match the pre's and start listening. It would be rather shocking if you heard a difference here, but would be worth knowing. But note which items are missing or revealed in PD.
5) One change at a time, revert your Marantz to the settings you always use, listening after each change. The objective is to determine where the sounds that got lost appear (assuming they were not present in the PD setting).
6) One change at a time, revert your Yamaha to the settings you use, listening after each change. The objective is to determine where the specific items get lost/revealed.
Note: My Oppo has a handy A/B loop feature that allows me to continuously loop between two points in a song so I can repeat the revealing spot in a song while switching between pre's.
Also, the linked switching device is listed as AV, but you can use the video connection for the pre-amp sub output!