Perhaps UHF was your problem. Above 30kHz, the impedance of the piezoelectric tweeter drops to almost nothing. But they do produce sound at very high frequency which is why you'll occasionally see devices like bug repellants with piezo's in them - they're running them way up high like that. You can see from the response chart in earlier posts, that these drivers have a peak around 27kHz. But they don't handle much power there.