I don't think you need just a preamp, if you plan to run 5.1 setup without AVR. You'd need a 5 channel power amplifier with a 5.1 channel preprocessor with Dolby decoding capabilities. For Atmos you'd need processor capable of more channels and amplification.
You're better off upgrading your AVR instead of investing in a multichannel preprocessor and Power amplifiers. Decent multichannel preprocessor are quite above the budget specified. Probably sell the Marantz 6009 (with added budget)and buy something newer and better (relatively)like SR6012 or SR 7012. Use the preouts to feed the Emotiva as previously.
You're better off upgrading your AVR instead of investing in a multichannel preprocessor and Power amplifiers. Decent multichannel preprocessor are quite above the budget specified. Probably sell the Marantz 6009 (with added budget)and buy something newer and better (relatively)like SR6012 or SR 7012. Use the preouts to feed the Emotiva as previously.