If 2-channel audio is important to you, any part of the AVR in the chain will ruin it. Period. Preamps matter as much as, and many feel even more than, an amp, and the preamp section of the SR8015 is garbage compared to any decent dedicated stereo preamp and will be a significant bottleneck to achieving better stereo performance.
The easiest solution would be to buy a good stereo integrated amp because you can remove the AVR completely from the 2-channel signal path by adding only one box to your system. To do this you just plug the front L/R pre outs from the SR8015 into the HT bypass (if it has one) or any unused line-level input on the integrated and move the speaker cables for the front L/R speakers from the Marantz to the integrated and you’re done. With this setup the SR8015 is completely out of the loop for 2-channel listening and only the integrated is in the signal path, and to switch between stereo and multichannel you just change input selector on the integrated — that’s it. Anyway, hope this is helpful and best of luck moving forward.
The easiest solution would be to buy a good stereo integrated amp because you can remove the AVR completely from the 2-channel signal path by adding only one box to your system. To do this you just plug the front L/R pre outs from the SR8015 into the HT bypass (if it has one) or any unused line-level input on the integrated and move the speaker cables for the front L/R speakers from the Marantz to the integrated and you’re done. With this setup the SR8015 is completely out of the loop for 2-channel listening and only the integrated is in the signal path, and to switch between stereo and multichannel you just change input selector on the integrated — that’s it. Anyway, hope this is helpful and best of luck moving forward.