seems to be a misunderstanding. the dac feeds an opamp - this is OK. then the same circuit feeds a tube - that is Ok, too. whether it's inserted, physically or electrically, is moot if the signal is output from both sources. think of a single SS DAC with two outputs, then one is fed directly into the next stage which may be an amp, and the other is fed into a tube buffer (cathode follower, or white cathode, or SRPP) then that is fed into the same or a different amp. never said "separate tube-based output circuit", in fact, let's requote: "The TubeMagic D1 comes with a vacuum tube pre-amp output stage in addition to the normal solid-state audio output. " semantics after syntax: "in addition" does mean "separate tube-based output circuit" otherwise how else is this designed? the fact, the word separate does mean "in addition" or a second signal output as I mentioned earlier otherwise if it wee only a single signal, then to get the other there would have to be a switch or "insertion" of the tube (or to be completely thorough - tube circuit) and if this tube buffer is incorporated into that SS DAC then it would be a good thing. it would seem that the issue might be this inserted tube thing, and it would make the capability/feature optional depending on some act by the owner which would make sense in using "inserted' to describe the intentional action as a result of selecting the "tube" function. and not sure what is meant by "circuit is by the same opamp" makes the tube portion irrelevant or superfluous or in this case, remarkable. in my case, the sound is different - the tube, to me, is preferably, if for no other reason, is that my music is much different than other folks, and the music is better to the point that my bi-amped system always has tubes for the treble side for it's richness. and is easily heard when switched out.