1) What creates soundstage and is there something about tubes that gives them this ability?
2)
How is the signal leaving a powered linestage better than the incoming
signal? I can answer that question for amps, phono stages and MC
step-ups, but beyond "attenuated" I'm still conflicted on powered
linestages.
3) Is there an argument that a lot of energy behind the
signal going into the linestage attenuator can produce better results?
If not, why does BAT have all that energy storage?
4) What new or
used tubed preamp less than 4k will give me back that compelling 3D
soundstage that extends in front of speakers without added bloat and
syrup? New Backert, ZOTL or Van Alstine or a used BAT or AR? I'd like
one that doesn't create a lot of heat.
6) Should I leave things as they are and look to speakers to address the soundstage issue?
1) proper phase relationships and bandwidth
2) It isn't. The thing is, what you are not including here is the effect of the interconnect cable. The fact is most cables editorialize- the have colorations of their own. One thing a decent line stage can do is minimize those colorations and so it can appear to sound better, because it does. But there is no question that it degrades the signal! Funny, huh? With regards to cables, it is easier to reduce cable colorations by going balanced. However there is a balanced standard that is not well supported in high end audio, so you will still be auditioning cables if your gear is balanced but doesn't support the standard.
3) No. But you are confusing (or the post is mis-worded) the energy going into the volume control (attenuator) with the energy used to power the circuit. They are not the same. The energy in the power supply of the preamp simply has to be as clean as possible so that the active circuits are not influenced by the power supply. Brute force is one way to do it, good regulation of the supplies is another.
4) If you want to avoid 'syrup', that is an aspect of single-ended operation. But a preamp can be single-ended and have very low distortion, in which case it will not be 'syruppy'. Auditions are important. We avoid syrup (2nd harmonic) by employing differential balanced circuits throughout our balanced preamps.
BTW, tubes don't have to have syrup, even though they are often know for a lot of 2nd ordered harmonics (example.: SET amplifiers). The harmonic distortions generated by any circuit dictate how that circuit 'sounds', since the are converts all forms of distortion into tonality of some form. So the 2nd contributes to richness and syrup while the higher orders (5th and above) contribute to brightness and harshness (in particular, the 7th contributes to a metallic quality). The reason some circuits have certain distortions and not others is very dependent on the topology of the circuit- for example whether tube or transistor, single-ended circuits will have more 2nd harmonic than a circuit that is fully differential.
Unfortunately the industry does not recognize how the various harmonics influence sound (even though we've known this since the 1930s). So the spec sheets will continue to not be all that helpful- you'll still have to audition the gear to see what works for you.