small amp, direct to 240x
Widget has written an Excellent thread titled "Ring Radiator Comparisons" that you really should read. Lots of good stuff here, thanks to his efforts, great test equipment, and contributions from other forum members that live near him.
It's fairly well known that the ring radiators need very small amplifiers due to their extremely high efficiency and the limited amount of UHF signals in music. 40 watts (re:8ohms) would produce 125dB at 1 meter on 2402's. A cymbal crash at this level could do some damage! How big is your listening room?
The ring radiators sound much better when directly connected to the amplifier with no crossover components or L-Pad in the circuit between amp and driver, so your triamp approach takes care of that. It's been suggested that the ring radiators are ideal candidates to be powered by little "chip" amplifier kits. The actual impedance of ring radiators are really about 12 ohms, so a high-current power supply is unnecessary, which keeps costs down. Large heat sinks would not be a requirement either, due to the brief transient nature of UHF signals. You could even build it inside the cabinet using a metal plate as a combination input panel-heatsink- base plate for the amp circuitry and PS. This would also improve damping and transient response since the cables to the tweeter would be so short.
One forum member here used 2404 "Baby Cheeks" horn loaded ring radiators from 8-10KHz on up to fill in where his big HF horns' response fell off and got ugly sounding. He reported that, at the recommendation of a JBL PRO Rep, he bought the appropriate capacitor to make a passive crossover that attenuated everything below 20 KHz at 6dB per octave and placed it in series with one of the leads going to the 2404. He said it accounted for the falling response of the 2404 above 12KHz, and that, due to the extremely high output of the 2404 ring radiator, they were still plenty loud enough to keep up with the big horn system. If your are triamping, installing or engaging CD horn EQ in the Active crossover on the UHF signal output would accomplish the same thing while still allowing direct coupling of the amplifier to the ring radiator.
The noise issue could be your DBX crossover, or maybe the gain structure in your system (signal levels from one component to the next from signal source through preamp to crossover to amp) could be out of whack. Balanced lines increase signal to noise ratio by 6dB and are immune to interference and noise problems along the cable, so use balanced cables between components wherever your equipment will accept it.
I don't think much of the DBX 223 or 234 crossovers. To me, they aren't super-quiet, and the signal degradation from input to output left me disappointed. Several bands I mix for have them in their PA's, and a few sound companies I have worked for use them, and I owned a couple of them for a short time. They work fine, but they just didn't cut it at home.:( AfterA/B'ing them against others at home and on the road, I never looked at them the same again...
The Rane active crossovers sound a helluva lot cleaner and quieter, and can be had cheap on the used ebay market. The delay/alignment feature is useful for time-aligning HF horns with MF cones, and they have provision inside for easy addition of CD horn EQ if you're the least bit handy with a solder iron. You can even transplant the time delay circuit from one output section (Low, mid,high) to another if need be, i.e. horn-loaded subs, or to delay the ring radiator to time-align it with the HF horn below it. The mod info is all on the Rane website, and easy to perform.
Ashly crossovers sound slightly better yet (I have limited experience with these), and have a couple other tricks up their sleeve that are useful in some cases, but they are much less plentiful and command a much higher price on the used market. I have half a dozen of the Ranes and am very pleased with them both in PA use and in my high-end home setup.