Yo 10KSLIDER:
Point one: If your mics need phantom power, make sure you snap that switch on the back of the MD-8. [I also use the MD-8]
If you're running your mics through a mic processor, like the ART MP, the ART has a phantom power switch that must be engaged or you will get nada from you mic.
Point two: You have a mic gain knob for each input channel or each channel you want to record from one through eight; so, if you need a little bit more gain, use that knob which is at the top, or thereabouts, of each channel.
Point three: If none of the above works, something is awry with your connections or the Electric company has cut off your power due to non-payment.
Cheers,
Green Hornet