For what it's worth, the newsletter that showed up in my inbox last night said that the 7500 and 7700 are entering production and should start shipping in early January. No mention of the 790, however.

I'm not sure how the Outlaw and Sherbourn amps compare directly. If I remember correctly, Sherbourn uses a "multiple monoblock" design approach (each channel has its own power supply), whereas the 770 and 755 used two toroidal transformers in a power supply shared by all channels. I'd expect the 7500 and 7700 to be designed similarly. If you don't want to wait for the 790 and wanted amps that match the 990, the 7500 or 7700 would be an option, but you're not going to gain any power (the 5/1500 being a 5x200W amp, just like the 7500). Both the 5/1500 and the upcoming Outlaw amps offer balanced connections.

Depending on how the resale value is on the 1500, there may be little or no price penalty to getting the package once the 7500 and 7700 arrive. I would expect there to be no drawbacks or penalties sonically from selling the 1500 and buying a 7500 or 7700 (and the 7700 would give you seven channels, allowing for an upgrade to 7.1 later), but aside from the option of having more channels and the aesthetics I don't know that there would be much benefit either.