What do you mean pure DAC mode ?
If you meant Pure Line Out mode then the dx340 has Pure Line Out
It is just the design of the 1 Bit DAC is unique and unconventional. That is in house by Ibasso alone.
Because it is In house self developed, for all we know, the DAC by itself coming off from 1 bit array, could be very low line voltage. Because of this very low line voltage, the Preamp gains will be needed. The Amp Gains on the 340 is PreAmp gain.
Line level gain has nothing to factor whether or not it is true line out. True line out is bypassing amplification process.
Other DAC or DAP already have high voltage level DAC line. Again, this is based on the architect of the DAC itself.
The 340 has very low LO voltage
2Vrms Max and or 1Vrms Max. That is with preamp gain. That further solidify my take that the voltage coming off the DAC itself is very low