Update power cables and interconnects.
Interesting, what Marantz "receiver" do you have that has XLR balanced outputs? I don't know of any receiver that has XLR outputs.
Another way to go, especially if 2-channel is important and the rest of your components are up to it, is to buy a good stereo preamp and incorporate it into your current system while still using your Marantz for multichannel duties. Using the preamp section of an AVR for stereo is a performance killer. Would be very helpful to know specifically what equipment you have.
Sorry to piggyback here but I have a Yamaha 1060 AVR that I'm using as a preamp with HT ability and a Parasound A23 Halo for my mains. I also have a Bluesound Node 2. Doesn't connecting the Node via coaxial and controlling the volume via it act as a separate preamp? How does going 'pure direct' play into this? I want to make sure I'm connecting everything the best way.
I've scrapped my 5.1 receiver for a 4.0 channel setup. I've substituted an ARC SP-6a (updated) for the front and ordinary stereo use; substitted an Audionics BT-2 for the rear channels, and eliminated subwoofer by using Thiel 3.5's which are flat down to 20HZ (with equalizer). The power amps are Outlaw 200 monoblocks, which have the advantage of being triggered by the audio signal, so the rears shut off when I'm listening to stereo only.
I used to think I couldn't live without matrixed surround sound for my phono, tuner, and tape sources, but I have found returning to a really good tube preamp more satisfying and I no longer miss it at all.
There are may ways to skin the cat with a little imagination.