I came across this thread talking about the NFB 29 (which is what you are considering):
https://www.head-fi.org/threads/audio-gd-nfb-29-single-ended-master-race-lol.732157/
Interestingly enough, he is using it through some AirMotiv speakers! lol. Lot's of good feedback on this NFB 29. One of the comments that stood out to me:
The nfb11 has some harshness to it in the treble regions that the nfb29 doesn't have... The 29 is more dynamic, more detailed and overall better. Much bigger sound stage, much better separation and imaging.
Though, if you spend more you get something better, obviously.