I don't usually agree with George Hifi but I agree that a tube has no business inside a Dac, for the reasons he listed and I will add that a tube is an open door for noise (EMI RFI) to potentially get inside the Dac and taint and mess with the conversion process. Don't get me wrong my system is all tubes and I love tubes but not my Dac, if I want tube sound I can get it from my preamp, the Dac must be neutral, shielded and detailed and have an option for minimal filters or NOS, I do pre processing on a pc and I can alter the sound signature with filters, modulators and such if I want to, or not. As a matter of fact my idea of a perfect dac is one which doesn't mess with the digital part much but has an EXCELLENT analog conversion stage.
I respect that designers implement dacs with tubes and there is a market for it but it goes against what I think a dac should be. In the case of the OP since he has SS a tube dac might help him to get that colored sound.
Perhaps someone could explain, I say colored because I still don't get the term tubey sound, since when tubes have a specific signature sound?, I get that if you are running DHT (300Bs, 45s, etc) you could get a specific euphonic sound but other triodes like the 6SN7 it can be very very neutral IMO similar to the sound of SS so it really confuses me when I hear the term tubey, maybe someone cares to explain.