I take it your comparing your U1 mini streamer -> Pontus II DAC -> Athena Pre with your Marantz Pre/Pro by placing the Little Bear between your Pre stages and your Amp(s)... Wonder what DAC chip(s) is in the Marantz? ESS? Sounds like an interesting comparison...
I did something like this a few years ago, although RCA connection based... My U1 mini -> Qutest DAC -> PremaLuna Pre won over my Yamaha Pre/Pro/Amp by a long shot... I eventually tossed the RCA switch and attenuated a pre output of the Yamaha and went direct into another input channel on the PremaLuna Pre... Mostly because the PremaLuna has a remote to switch between input channels and it does make the Yamaha signal sound a little better because tubes... No HT bypass for me...