[quote name='BigRedX' timestamp='1386839921' post='2305098']
If that's the case, then it's piss-poor design. An effects loop should be either series or switchable. I can't think of a single instance when parallel only is more useful.
[/quote]
Markbass are parallel by default, switchable with a jumper inside the case. I assumed that most amps would be done this way, but it seems that I'm wrong, and that series are more common. Apologies. From the circuit diagram it looks like it is indeed a series loop. The point about the line level signal still stands.
One reason given is that, even if something fails in the loop, you still get a noise coming out. Another reason is that it may allow to you blend the clean and effected signal, something which some older effects units may not offer.