Rob, welcome. In my view, since the 3490 is a superbly flat amplifier, connecting it to the pre-outs of the tube amplifier can serve only to degrade the sound, not "add" sound quality of a good sort. So yes, connect the 3490 directly to the DAC output.

The 3490 does have pre-outs, so if the tube amp does have an input for a line-level output(I'm not familiar as to whether it does), you can connect it to the 3490 pre-outs. Incidentally, you might also try simply using the headphone jack on the 3490.

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Enjoy the music, not the equipment.