Update 6/2/18: I completed the discrete op-amp modification after I got the replacement diodes. You need to replace Diodes 11, 12, 13, & 14 with 16V Zener diodes. If you fail to do this, your op-amps will be getting +18 and -18V (36V total). This is a pretty high voltage to use on op-amps, these days we usually see 15V to 16V. If you skip this step, you should at the very least contact the manufacturer of your op-amps to ask what the recommended max voltage is (or check the data sheet).
I have been running the Kenwood C-2 with the Burson Audio op-amps for about 2 weeks. Overall I have been very impressed with an already excellent pre-amp. I noticed better high frequency extension (without being fatiguing on the ears), surprisingly the low frequencies also seemed to be clearer with less distortion during bass tests (I used DSD256 reference disks).
One of the other projects moving through my workbench is this sweet combination of excellent bare bones pre-amps using discrete op-amps from Burson Audio! For now I just have a few pictures to share, but stay tuned to hear more about how this pre-amp will play a role in my reviews and how it is used with normal amplifiers.
Note: Hallman Labs will soon have a balanced in headphone amplifier! It was time to upgrade from the sub $100 Schiit Audio Magni2, but this is still a great headphone amp for the money. One of the best starter headphone amplifiers you can probably find for the home user.
A shot of the V5i op-amps in the tone stack/EQ circuits/filter circuits.
More to come later, including a recap and a few community sourced mod ideas.
See one of the threads on DIYAudio here: http://www.diyaudio.com/forums/solid-state/184318-kenwood-basic-c2-preamp-mods.html