Malwarebytes is not geared for MBR infections - it may indicate TDSS variants though. You must bear in mind that MBAM complements Avast inasmuch as it looks for known file names as opposed to signatures and behaviour which is Avasts forte… The packing and disguising of malware now is an art as the race between AV programmers and malware programmers hots up

Did you upload the dropper file to Avast for inclusion and analysis ?