Quick Neurochrome mod686 update for anyone who has this amp or is considering it. I changed out a little surface mount resister to increase the input gain from 26db to 29db and that seems to have had a dramatic impact on this amps performance. I am using an Elekit TU8500 preamp as a unity gain tube buffer and the combo is really good.
In your setup, you may have just needed the extra gain, although your signal to noise ratio will be slightly worse. It’s not a big deal with the 686 since the base SNR is sooo low anyway. It also depends on the output voltage of your source and sensitivity of your speakers.
For example in my setup with a 4-5V RMS source, 96 dB speakers and no preamp (I used to own a TAPX though), the 20dB gain version of the 686 is best. I removed the same SMD you changed and that dropped the gain from 26dB to 20dB.
It’s all about adjusting things to your liking. One thing is for sure, the 686 isn’t the bottleneck if transparency, neutrality and detail are what you are after.