[Daren]

Brendan, a master harmonica maker/player says this:

It sounds like the dreaded Helmholtz Resonance Coupling issue. Happens on high reeds in harmonicas also, when the comb chambers are too large and/or the reeds are a long way from the mouth. As one answer suggested, it’s probably your enlarged chambers that are having the bad effect on the high reeds.

Put a few high reeds back on the original setting block: do they sound well? If so, redesign your block so that you only have the enlarged cavities where needed, on the long low reeds which have a considerable swing. High reeds will have very little swing, so don’t need the compensation.