DHT preamps produce the most 3D /holographic soundstage.Probably because those same tubes do the same thing when used in power amps.This statement is entirely false. Soundstage is not created by how the filament interacts with the input signal! And we can also show easily enough that SETs don't rule the roost when it comes to creating a 3D soundstage.
To get the soundstage right, the circuit has to have bandwidth such that phase shift does not exist in the essential regions where it makes the most difference to the human ear. For the most part this is the Fletcher-Munson curve frequencies, about 3-7KHz. If the preamp lacks bandwidth past 30KHz its not going to get this right.
So a DHT preamp would only work if its bandwidth met this criteria. It would be doing this because of bandwidth and low distortion, not due to the DHT.
I recommend to anyone that is interested in getting to the bottom of this sort of thing to get a decent set of microphones and a good studio quality recorder. Go out and make a good, 2-mic recording that you can stand to listen to over and over- and then produce it on LP and a digital format. In the case of LP you don't have to produce 500 copies or the like; a test LP is fine. Now you'll have a real reference- because you were there at the recording session and you have the master tapes or master file.