Interesting questions. Personally, I'll stay with my 990, but it may be relevent to look back at the 1050. I was hardly the only person to buy a 1050 and use its amp section while saving up for a separate amp (I bought a Model 750 about a year later), and the 1050 worked very well as a pre/pro from the time that I got the 750 until I replaced it with a 950 the following year. Getting a 1070 now and using it as a springboard into separates makes some good sense, and I'm sure some folks will do so.

The question of whether the amp will drive a 4 ohm load has less to do with the 8 ohm power rating and more to do with the restrictions that tend to come hand-in-hand with receiver amp design. There is a "4/8 ohm" switch, so it will probably handle some 4 ohm loads, but I'm always wary of asking a surround receiver to handle difficult-to-drive loads that often are rated at 4 ohms. After all, the ratings on speakers are generally a "nominal" value, and some low-impedance speakers that get rated at 4 ohms can dip down to 2 ohms or less fairly often. A speaker such as that would probably be best served by a separate power amp.