with 6dj8 and 6sn7 adapters for the mhdt units, one has a very very broad range of tube choices to roll, and both tube types have long illustrious histories and were made in various parts of the world to boot
you will get somewhat lower gain from some variants(esp. older 6sn7s), but since the tube is used as an output buffer only, that will not impact the dac's output voltage too much
for all tube rollers of orchids, stockholms, istanbuls and the like, just be careful -- be very gentle and use a steady and firm hand when removing the tube from the adapter, find the right orientation where gentle rocking pressure will loosen and release the tube pins, and especially when releasing the adapter from the socket on the dac's pcb board... the board is somewhat flexy and there are examples of folks breaking connections between the tube socket and the board below - jian at mhdt has been cool about sending replacement boards in the past, but one cannot count on his goodwill being extended indefinitely
you will get somewhat lower gain from some variants(esp. older 6sn7s), but since the tube is used as an output buffer only, that will not impact the dac's output voltage too much
for all tube rollers of orchids, stockholms, istanbuls and the like, just be careful -- be very gentle and use a steady and firm hand when removing the tube from the adapter, find the right orientation where gentle rocking pressure will loosen and release the tube pins, and especially when releasing the adapter from the socket on the dac's pcb board... the board is somewhat flexy and there are examples of folks breaking connections between the tube socket and the board below - jian at mhdt has been cool about sending replacement boards in the past, but one cannot count on his goodwill being extended indefinitely