I've a vox pathfinder 10 and its speaker got blown a while back i tried the amp on clean and it sounded almost as normal but distortion was definitely dead i tried connecting my mfx to it and dist was just like before it was blown only difference is much more noise (gone by touching any metal part in guitar )
When i use my dist pedal the distortion is still dead
So .. What's this ?
If the speaker can't give good sounds why is it good with the mfx
Could it be that the amp's preamp is ruined or something ?
The distortion pedal probably sends the amp to natural overdrive, which is impossible to correctly do considering the condition of the amp.

Your multi-effects most likely sends an already distorted signal to the working clean function of the amp, so you're just amplifying that signal.

Also, replace your speaker/get a better amp.