The UA plugs are great sounding and you can get a good mix from them, but I don't think you really need to spend on the Apollo if you aren't recording full live performances. If you are doing overdubs, the M Box is just fine.
As far as the rest of your setup goes. Spend the money you would save on the Apollo on room treatment and better monitors. Room treatment being extremely important, it will make a huge impact on how your mixes will turn out.
Feel free to call me if you would like to discuss more.
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andrew _ malloy @ sweetwater . com
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