Maybe you already thought about it but, just in case …. : on the DP-008 (and other Portastudios), the *input* level of a source is not controlled by the corresponding channel volume button (which is for playback).
it is the “Input A” / “Input B” volume buttons that dictates at which volume your recorded punched-in / out will be.
So, you have to compare the input level meters (A and B at the leftmost positions on the 008 screen), with the already recorded material on the track you’ll do the punch-in/out, and also the output levels L and R shown on the rightmost positions.
Adjust Input A / B « trim » input volumes accordingly. If you do not, evident level difference will be heard on that particular track with punched-in insertions.