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.
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.