The Markbass heads operate with a parallel effects loop as standard - hence the 'blended' effect you noticed. They can all be switched to series loop via a jumper inside the amp, but they don't have an effects loop blend, per se.
It's quite easy to find and change (if you fancy doing it yourself) - failing that, your local Markbass dealer should be able to sort it out for you.
You will lose the LM2's clean blend tho, although the POD should be able to compensate for that.
I have an SA450 head, and decided to leave it in parallel for exactly the reason you mentioned - but i agree - some sort of wet/dry mix control would be welcome on the next revisions of their amps