Did you try setting the 'duration' on the IR code to see if it affects the time to send? The extender for the NG series also has a High/Low switch on it if I recall correctly.
Are you using the RFX 6500 or the older RFX 6000 (no power settings)?
I am hoping that you say RFX 6500.[Link: files.remotecentral.com]
If using RFX 6500, per the manual, there are 5 DIP switches on the bottom. By default, all are On.
Switches 1 + 2 control
Dual IR emitters 1 - 2 Power Level - On likely High Power. Off is lower power
Switches 3 + 4 control
Dual IR emitters 3 - 4 Power Level - On likely High Power. Off is lower power
Switch 5 Controls
IR blaster On/Off (you can use the IR Blaster without plug-in emitters)
Give this a try. Start with the Blaster. Then try an emitter cable stuck to the front of the Denon. I don't recommend the wired cable as it may be introducing a ground loop (hum) that is preventin the signal from working.