Ok - I think I figured out the culprit.
Actually, potentially there are two.
1. I was routing the signal from Reaper only to the left input channel (OUT 6/7). After routing it to both (OUT7/8), the tracks look (almost) identical.
2. The other thing I noticed (as per suggestion of
@Chromatizm) is that somehow (probably due to me going back and forth between firmwares) the input levels were reset to 50% in fw28.00 instead of the 8.5% default that I had.
So, thank you all for the help and apologies for my user error.