Stacking DAC's..........

Wouldn't I be "stacking" DAC's when I run run an external dac with the pre amps I use for my amps?
Kind of like using a external crossover with a powered sub with built in crossover?
I have 3 pretty good pre's with EXCELLENT Dac's.
My question is, if I run an external Dac into a digital preamp with its own dac, isn't that "stacking" Dac's?
External dac into pre dac, whats the point?
My pre's:
B&K Ref 20 (20bit)
B&K Ref 30(24bit/96)
Atoll PR 5.1 (24bit/96) AWSOME BURR BROWN DAC'S!
All of these pre's have excellent DAC's.
So whats the point of having an external DAC other than USB input?
To strictly answer your question:
1. If you use the ANALOG outputs of an external DAC to feed the ANALOG inputs of any of your preamps, you won't be stacking DACs. There will be only one D/A conversion, in the external DAC and the internal DACs of your preamps will be bypassed.
2. If the external DAC has "digital loop" and you feed a DIGITAL input of you preamp with S/PDIF signal from the digital output of the external DAC, you won't be stacking D/A converters, for the purpose of such a loop is to allow further processing in the digital domain (It may be worth to consult the documentation of the external DAC, for assurance). The D/A conversion will take place inside your preamp.
Now, for a non-strict answer, in my humble opinion, #2 will be a misuse of an external DAC if all you want is USB and want the D/A conversion to happen in your preamp. You may go for a USB/SPDIF converter instead.