Finally decided to do something about the cartridge that was supplied with the MMF-5, a Goldring 1012GX.
I picked up a Grado Statement Platinum from musicdirect and got it installed this evening. Even though it's not broken in yet the clarity and upper frequency extension in comparison to the Goldring is excellent. The bass is pretty soft right now but I'm sure it will improve over the next few weeks as the suspension breaks in. It's noticeably better now than it was a few hours ago. The Goldring has always had a channel imbalance, not so with the Grado. Images are nicely centered and don't wander.
Also, if you happen to own an MMF-5, consider replacing the cable from the arm to the plugs. I thought I would re terminate this cable with some bullet plugs while I was working on it but after clipping a plug and stripping the wire I knew I had to replace it. The factory supplied cable is poorly shielded and packed with some type of powder. Not sure what the powder was, possibly some dry lubricant used in manufacturing, I really don't know. It was the poor quality braid that made replacement desirable from my point of view.
I had some miniature coax on hand that seemed appropriate (32GA, 1mm OD with a nice shield) and decided to give that a try. Turns out that changing the cable on this TT is easier than it appears. Once the arm is out there are two small set screws holding the cable mounting plug in the bottom of the arm. Just loosen them up and the assembly can be pulled out easily. A few minutes with a small soldering iron and a hot glue gun (strain relief) and you're done. I terminated the other end with Eichman bullet plugs.
I'm fairly sure the cable change is responsible for part of the new sound but in any case, the upgrade was well worth it IMO.