Rotel amps are definitely not one that is recommended for 2 channel listeningPreamp- Marantz AV8805: $4500
Power amp(2 channel)- Rotel RB1590: $3700
Power amp(5 channel)- Rotel RMB1585: $3700
I’m a little bit confused ? What am I missing here? The OP got $11,900 worth of gear and his gear is NOT recommended for 2 channel listening!? We all should be so lucky!
I say go for exactly what you want - the Audio Quest Thunder at around $900 each is perfectly reasonably priced - for YOUR budget. And do NOT believe the naysayers that say there will be no improvement in sound quality. You will get quieter and blacker backgrounds and better and less fatiguing sound coming from your speakers.
Also, I think it maybe a good idea using all Audio Quest PC’s as they will have the same sound signature and blend in well with each other, especially if you are planning to get a Audio Quest conditioner at a later date.
And one more thing, don’t stop with other upgrades and tweaks. Damping, IC's, speaker wire and fuses to name a few.
Sheesh! What a tough crowd!