If you want to use the avr's dac and bass management you're just better off using that unit and getting a separate phono stage/pre-amp for your tt. I'm not quite sure what you mean by "U-turn turntable without the amp going to a cheap preamp". You mean the U-turn has a cheap built-in phono stage or something? You can buy a reasonable phono stage for less than $100 easily enough, altho a built in one on the U-turn could be adequate.
As to the two sub pre-outs, in this case no different than having one sub pre-out and using a splitter since the two pre-outs aren't separately adjustable for level/delay (i.e. the avr basically just has a splitter built into the unit). The bass management in the Rotel should be better than the lack of same in the Yamaha, tho. Altho depending what subs you have, you might have another workaround, too.