To follow zac's line of thinking - what particular type of advice were you looking for? I've built more than a few pairs of every model of FrugelHorn, as well as at least a couple of Scott Lindgren's larger manifold designs ( i.e Aiko, Maeshowe / Victor)
Well, the need for tweeters would depend on the main full driver used - in the case of numerous pairs of the three different models of FH, and two models of tall bifurcated designs I've personally built, almost all of the drivers- Fostex and Mark Audio- have been quite fine to my ears without any help. But then after almost 25yrs of inconsistent adherence to hearing protection protocol in a commercial woodworking shop compounded with general age related loss, I probably have not much acuity for raw test tones in the top octave, and don't know exactly what I'm missing. Of course the same will be true when I'm listening to a $25,000 pair of commercial design in our remaining local high-ish end audio emporium.
Chris, not a Frugel-Horn but Haruna for the FE166.
I have not heard these, but Bernie built a set of Victors. To get bass out of these drivers you need a horn, the Victor certainly supplied that in spades. I did not find the need for a tweeter, but if you have bat ears you may not find it sufficient. A horn is recommended.
The driver does really want a high output impedance amp (we were using a 2A3 SE).