Just thought you might be interested in my recent experiments with this super cheap horn unit from skytronic via Blue Aran. The driver was £4.40 and the flare £5.00 so a tenner for a decent size horn. The sensitivity doesn't tally with the manufacturers claim; I measured it as 103 dB/w with filtered pink noise, still pretty good.
I also experimented with attenuators and accidentally with a crossover. Piezo tweeters are capacitative so putting a capacitor in series acts like putting a resistor in seried with a 'normal' tweeter. Being in to much of a hurry to make any measurements I guessed its capacitance and by trial and error tried a 2uF capacitor and achieved a 3dB reduction in sensitivity. This sounded great coupled with a 12" speaker but gave a bit of harshness where the two units overlapped. I added a 10ohm resistor and this cleaned things up nicely though I only did this to protect the amplifier at high frequencies.
This gives a lovely sound with my bass though I only did it to make a PA speaker.
I'd recommend anyone thinking of a cheap way of adding a bit of top to a speaker to look at these.