I don't know which Kush varieties you have grown that had a small yield, as far as I know Kush varieties are associated with a pretty nice yield.

Anyway, I would have to agree with m.g. there PrisonerInGlass. But to answer you question, Try some Mazar, Diesel (preferably #2, thats exactly what your looking for), Hash Plant, G13 (if you can find it) all of these strain are extremely potent, none have a THC content below 30%, well the Mazar might I can't remember...
smoky mcpot 05 Reviewed by smoky mcpot 05 on . Small yield-high potency strains? Anyone know of any? I want to breed some THC-laden varieties with varieties of larger yield. Probably going to use Big Bud for the large yield. Rating: 5