Separates will generally offer at least somewhat better sound quality (although it all depends on what receiver is being compared to what set of separates). Separate amps can typically provide better sound than receivers because their design is less constrained by space and shielding from other signals, allowing for a larger power supply and better cooling by means of bigger heat sinks. For example, the 7075 is rated at only 10W/channel more than the 1070 but is nearly the same size - and if it is anything like its bigger siblings, there's not a bit of wasted space inside. The other benefit of separates is the ability to upgrade only the processor later, as new processing technology and new formats take the stage - good amps last a long time and will not be made obsolete by changes in the industry.

The 990 will give the most audible improvement, I suspect - the 970 and 1070 are so closely related that the only difference between a 970/7075 and a 1070 (aside from the ability to upgrade the 970 later and keep the 7075) is the larger power supply and slightly extra power for the amps.