Rendered drum tracks have lower volumes

Hi I loaded in drum tracks (10 per song roughly) sent back to me from the drum recording studio, put them into their own new projects in Reaper and performed some further edits. The volumes were always set to 0 as were the master faders. After these new edits, I rendered the tracks out individually once more (to my desktop actually) as 'final edits', again using 0db settings. However, when I loaded these rendered tracks back into Reaper to check what our mix engineer will get to work on, I saw that the track volumes are significantly reduced. I don't know by exactly how much, but the waveforms (just by eye) are about half the size of the originals. This is a bit of a worry: anyone got a solution, or know how I've done this?

Hello guys I see that I have 'master mix' selected at the top. I don't know anything about 'pan law' and cannot see a dialog box for that in the render screen.

I still think Glue is the best option in this case.