So I just tried the effects return with a couple preamps (Origin Effects DCX and Bassrigs, and the MBD3). The tone is neutral like you said, so it does look like the low voicing is mainly the internal preamp. However, as I suspected, it also sounds a little tin-can-ish, as other effects returns I’ve tried over the years. I prefer the normal front input.
There’s nothing wrong with the eq at noon, it’s its own thing which is probably much appreciated in certain contexts, and it’s easier to ease off the Bass eq than add body when it’s needed. An outstanding combo overall.