CPU power is abundant on the Macbook Pro, so exploring ways of delivering live broadcast stream directly from the same machine being used for mixing. The Shoutcast DSP is available, but appears to need an input from a file - live streaming disabled? Streaming directly from Reaper is possible only on Windows, since it uses a dll file and there is no Mac port. Then found this thread of posts about BUTT (Broadcast Using This Tool) and Soundflower.
In my initial trials, I was not able to get the hang of it, how exactly to use Soundflower to route the audio to both the US-1641 hardware outs as well as the virtual out to BUTT. Trying to create an Aggregate Device as suggested in Applications -> Utilities -> Audio Midi Setup.app - found that the US-1641 cannot be aggregated with other devices due to latency and driver issues. Thought about putting a physical cable from one of the Tascam US-1641 outputs to the built-in input of the Mac, and then googled virtual cable mac audio to come across Jack OS X.
With Jack's virtual inserts, I don't even need to use up an extra physical output. Jack is configured with 16 in and 4 out, mapped to the physical interfaces on the US-1641. Reaper uses the Jack Router as audio device, everything comes in automatically in the correct tracks. Had to restart Reaper once when it was first set up, that's all. Unmapped the physical inputs from BUTT's virtual inputs so that it receives only from the Insert from Reaper.
Edit: Just some caveats:
- Tascam US-1641 control panel needs to have inputs 3-4 made digital outs manually every time after booting.
- Order of startup is important. JackPilot - Jack - Butt - Reaper - Routing window - Load Saved routing setup is the order that seems to work fine.