Tiger musky may help. I would only use tiger musky and not northern pike though. Northern pike may reproduce and you don't want that...
If you stock tiger musky, keep it around 1 fish per acre. So you'd want about 7. Make sure the fish you stock are at least 12" long to avoid being eaten by the bass. There is a likelihood that lone northern in there already may eat your tigers before they get much size to them. Let's hope not...
The reason you crappie are doing so well if that the small abundant bass are keeping their numbers in check. As the bass population thins out, you can expect the crappie population and other panfish numbers to grow but not get as big as before. More competition between panfish means decreased growth rates. So if you are looking for improved bass fishing, then add the tigers. If you're more focus on the panfish, leave them out. Take a look at this link:
http://www.bassresource.com/fish_biology/pike.html Just substitute tiger musky for northern pike as they essentially act the same way in ponds...
Also, be sure of the legality of stocking the tiger musky.
If you decided to add tiger musky, after 3 or 4 years they should really start to do a number to the smaller bass. As the bass numbers decline, you can expect their growth rates to increase. At this point you can consider adding an additional forage fish to the mix like golden shiners. However, getting golden shiners or any other soft rayed foraged fish to take hold in a well established pond is VERY difficult.
