Just going by a level indication really doesn't tell you much. They can be out of calibration and could be adjusted to different level. Now if you receive more stations on the MD and with less noise then your Meridian is weaker. You can't just go by a level indication on two different tuners.
Had to google this. Not sure you will get much support here for a car radio. Doubt you would be able to order any specific Pioneer parts new for this unit. Best bet would be a used unit on Ebay for parts.
Usually go through the alignment procedure. I often see when the MPX (Stereo Decoder) adjustments is off and the MPX is unable to decode, it will be at a lower volume. Really easy to see with a signal generator and oscope.