That's pretty much the nature of the beast - it's tough to find mallets that will work throughout the entire range of a marimba, especially when using ranges beyond four octaves.
Generally, if I have to cover a wide range with one set of mallets, I'd go with something medium-hard. The soft mallets that work in the low end really tend to wimp out in the high end, and the mallets that speak well in the high end tend to sound crass in the low end - and I'd be nervous that really hard mallets would do damage to the thinner bars found at the low end of a 4-1/2 octave marimba. If a true medium mallet won't do it, I'll tend to favor a slightly harder mallet, something medium hard, just hard enough to speak well in the upper end. I've found it easier to learn to control a medium hard mallet in the low range, but it's almost impossible to get a mallet to speak if it's too soft for the upper range.
You mentioned Balter mallets. I'd suggest checking out the Shadow Series mallets http://mikebalter.com/shadow_series.htm. Those are the ones I use if I need to cover a wide range and there's no opportunity to switch mallets to suit a given range. They're wrapped tightly enough - and are hard enough - to still speak in the upper range, but there's a fair amount of mass to the cores, which helps the fundamental tone come through in the lower range.
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