I have the Tubemagic D1 (not the plus). It's very good. In direct comparison, it is not quite the quality of the O2/ODAC (which is comparable to the Modi/Magni). The ODAC/O2 has an edge in SQ. I also compared it with the FiiO E17, and found the Tubemagic to be better.
However, you wouldn't know the difference if you don't hear the Magni/Modi in direct comparison. The Tubemagic would sound good. I'd say the Tubemagic is just a good a value as the Magni/Modi given all the extra functionality.
Also, this model is not really a tube DAC. The tube is only integrated into the audio chain on the pre-amp outputs. The headphone output doesn't even go through it. So you'd only hear it with speakers or a separate headphone amp plugged up to those pre-amp outputs (but not the line outputs). I'm using the pre-amp output with a set of Energy V5.1s and an Indeed TA2021, and it sounds quite good. However, if you don't like that, you could always use the line outs on the Tubemagic and use the volume control on your Sherbourne.
You are also right. You'd have to get the Schiit Asgard and Modi to get pre-amp out capability to be able to use the volume control. There's not much signal loss in going with a splitter instead. I think it's supposed to raise the noise floor 3db by using a splitter (could be wrong), which is not significant.