I will leave experts to cover Marcelos' points 1 and 2, but I have lots of experience with 6922s.
I tried many 6922s in a Sonic Frontiers phono stage and found only one good one, the Mullard CV2493. Note, not CV2492 which is nothing, like as good. Later, I had a number of CD players using 6922 and found the same; the CV2493 gave good results. Here's some personal views. Do look up Joe's Tube Lore on Tube Asylum on the subject.
New 6922: a Cayin cd player came with current production EH; when I put in CV2493's the sound quality went through the roof. I don't know much about other current production but I doubt if they are much better. So for top sounding 6922, forget current production.
Russian 6n23p: some people recommend these, I don't. Thin sounding. Tonally poor. Perhaps a bit better than current production, but not good; perhaps better than FET or most op amps, but a good BJT might beat it.
Various other NOS, e.g.Philips; typically a bit better than the above but often a bit thin sounding.
Mullard ECC88, CV2492; generally a fuller sound, reasonable tone but a bit mushy, not great dynamics or rhythm, somewhat musical but not great, a bit lifeless. CV2492 not too bad.
Tungsram: there are several varieties; I found one type that was very good, full bodied, musical, natural, a touch of sweetness, similar to (but not quite as good as) :
CV2493. Full bodied, naturally sweet, good tone, good dynamics, lively, musical, detailed and transparent without harshness; excellent (for a 6922; a decent 6SN7 beats it though). By far the best.
I used my Atlantis in a decent system, with a good valve amp (using ... guess what ... 12SN7s) and Audio Zenith PMx2 phones. I also used a Lector CDP707, a really fine player. I'd say the Lector is better, with a fuller sound and more transparency and detail than the MHDT, but the MHDT has a gorgeous midrange, superb for vocals. So I listened mainly to the MHDT.
I tried about 3 good 5670 and variants in the MHDT, including WE. But I had a definite preference for the CV2493 with adaptor; hard to describe the sound but I'll try. A fuller, more solid sound, more powerful; a bit more detail; it's hard to say if it was sweeter, but very natural sounding and no harshness. So it stayed in!
I hope this helps. Remember it's all personal opinion, my systems, my ears. However, my high opinion of CV2493 was confirmed, in different applications, by a golden eared friend, and by Mike of Astin Trew when I had a demo of the CD player at his home where I put in a CV2493 ... and he wouldn't let me have it back!
On other 6922s, I pull no punches and sorry if have trashed some people's favourites; I just suggest you try my suggestion before you flame me!
The issue with the CV2493 is of course availability and price, but they are worth it. I'm lucky, I got a reasonable supply many years ago; sorry, not for sale ...
Now for the relevance to MHDT.
I doubt if the typical 6922 will give much improvement over the better 5670 and variants; but it's worth trying. However I do know how well the CV2493 works here. As it's well balanced (does everything well) I can't think why anyone would not like it - but personal taste does come into it, there's always a chance you might not like it.