Hello ab1vl and welcome to these forums. You'll find many helpful people here and a lot of useful information. Make sure that you:
get the excellent User Guide by Nicholas too:
http://www.cockos.com/reaper/userguide.php,
check out Kenny Gioia's great video series:
http://www.reaper.fm/videos.php,
and the Groove 3 video series:
https://www.groove3.com/reaper-training-video-tutorials.
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The red streak is just an indicator of the MID note velocity. It's on the right-hand side of the meter as that's where the devs put it. It is not the right-hand audio channel.
Any thump is created by the plug-in, not by the MIDI itself. For other synths try LinPLug's Free Alpha or u-he's Zebralette or Tyrell N-6.