hello again,
sorry for late response, I've been converting my PC workstation into Hackintosh (getting ready for Sonar OSX Alpha heh), unbelievable success so far with El Capitan btw,
switching AudioSnap offline render mode to "same as online" did the trick (had to apply for whole project though, somehow couldn't make it work on individual parts, but that's not important),
here's waveform image comparison for reference:
from top to bottom:
1) rendered by "same as online"
2) recorded via RME TotalMix FX HDSPe AIO Loopback (AES)
3) rendered by default settings "radius mix"
I'm still trying to understand how come so I never really noticed such audible problem, perhaps this time the drummer and bassist were almost precisely phase-off (didn't audiosnap bass, only drums) so it could have resulted in such big difference...
big thanks to everyone, especially Lance Riley for pointing me the right direction!
fingers crossed for bright Sonar future :)