You will need a phantom power for the mics AND a preamp in front of the UA-5. The rca jacks are line level and do not provide amplification for a mic level signal.
Ummmmmmmmmmmm. I don't think so. The UA-5 is a preamp, and will power one of the sets of mics. You either need a phantom power source, or another preamp, but not both, to power the other mics going to the RCA inputs.
The UA5s RCA is controlled by a Volume knob, not a Gain knob.
You may want to set Volume at a known level and adjust Gain with a PreAmp. Or you could use only PH and the Volume together for the same purpose.
You want to think of the RCA inputs as an Analog Line-In (like of other devices), but with a Volume knob. You want to have a complete Mic set-up that will output Line-Out via RCA. However you achieve that is up to you.
I'm not sure I would rely on the Volume knob to get good results as it operates both channels equally, and is hard to adjust or even reach. I think a PreAmp will give better results.
Also, the Oade Plus mod made these RCA connections "cleaner". You may want to seek out a Plus Mod UA5 if you intend to do a lot of 4mic or Mat recordings using the RCA inputs. Its worth it...
Terry
(Former WMod+ Junkie)