in my case, I'm using the newest version of the darTZeel NHB-18NS as a preamp with the MSB Select II. I've not yet tried the Select II 'direct' to my amps. for a number of reasons, I suspect that the dart pre will perform better than direct. first off, i have 8 meter interconnects between my sources and pre and the amplifiers. second, i use the BNC 'zeel' 50 ohm interface between my pre and amps which has advantages over any conventional interconnects and is particularly synergistic in my system. thirdly; my dart pre is battery powered and extremely low noise and the volume attenuator in the dart pre is exceptional.
even with the previous version of the dart pre, the dart pre seemed superior to going direct from a source. 5 years ago i had some very high level headphone gear, including the
Headamp Blue Hawaii Special Edition with it's own volume control, and the Stax 009 electrostatic headphones. i figured it would be better to run my Playback Designs MPS-5 direct into the BHSE......which i did. but then i also tried it into the regular source plugs on the dart pre and out the fixed output of the dart pre. it was not close; the dart pre in the signal path was much better. and this is with likely the most detailed of any headphones....and very short RCA outputs from the dart pre into the BHSE.