TheFirstWard wrote:If I did purchase a USB interface, would that replace the phantom power supply or would I still need both.
Most USB microphone pre-amps that have XLR inputs also have phantom power (but check the specifications to be sure).
Although USB can supply power to run USB audio devices, it is not idea for several reasons (noise, stability, adequate voltage / current, ...), so in choosing a USB audio device it is generally better to go for one that can be powered from mains / wallwart / batteries rather than relying on USB power, though this adds a bit to the cost.
TheFirstWard wrote:I also ordered a reasonably priced used Blue Yeti from Amazon. Is that a better bet than the MXL 990 setup?
I've not tested either myself, but I would expect that the MXL 990 with a reasonable USB mic pre-amp would be significantly better than a Blue Yeti.
Inexpensive USB mic pre-amps are available from Behringer, ART and others. Price wise, you'd be looking at 'from' about $40 for a USB pre-amp (USB powered) or from around $90 for one that has its own power supply.