John,

Having used external headphone amps in the past, this is the first I have heard that you can use the headphone jack from the 990. The ones I have used only had the L/R RCA inputs receiving its source from a tape loop from the preamp. With that said, I suspect that you can use the headphone jack of the 990 and receive the Dolby effects through your ext. headphone amp and only this way. This way you can also use the remote of the 990 to remotely adjust the volume. I haven't seen an external headphone amp that I have used that has a remote. The ones I used required you to turn the volume knob on the unit. Probably the builder of your external headphone amp suggests you to use the tape loop because it is a fixed output signal and is not "processed" like the signal will be via the headphone jack potentially degrading the sound. I would suggest trying it both ways and see which sounds best for you. Currently, I am satisfied with the sound out of the 990's headphone jack via my Sennheiser headphones. Like Gonk said, once you engage the headphone jack, there will not be a signal found in the other outputs.