Sure. The effects loop on all Markbass heads is in parallel with the clean direct signal. That means that if there is a break in the effects loop you still have the clean half of your signal; however the total signal level is halved.OP's discription of problem to me points to fault with Amp and not sure about loop causing half power being parallel MB claims if pedal in loop causes problem input Signal will remain the same ; can you explain
PS; I would recommend getting Amp checked by Service personnel or bringing to store and plugin Cab section to LM 111 OR IV for comparison or even any 300wt into 8 ohm amp.
If the effects loop sockets are never used the contacts can become contaminated leading to an open circuit in the effects loop. I'd always recommend going through non invasive checks before involving a tech. In this instance it's not 100% there is a fault as the user is new to Markbass amps. However, if it turns out nothing improves the situation then a tech is the correct route. Or a return to the seller.