Garage Power is low...any way to boost?
The outlet in your garage will either be 15A or 20A, depending on the upstream breaker (hint: it's worth your trouble to find the breaker box given what you're trying to do).  This equates to up to 1800VA or 2400VA for a resistive load (less for the loads you're running).  

If all of your man cave gear is plugged into the 875VA UPS that you linked, that is the limiting factor, not your house wiring.  Get a larger UPS, run two in parallel and have them run different loads, or just use a surge suppressor and skip the battery backup for your less critical loads.

From there, you could also determine if you have multiple circuits supplying power to your garage, but you're a long ways off of having to do that.