I've never been satisfied with these original stepped attenuators. So I decided to investigate whether there is a better to replace with. Decwares attenuator caused disruption of both units when adjusted. Similarly, the sound was not pure enough. I found a better attenuator with same characteristics  at http://www.hificollective.co.uk/ , which is a much-publicized, TKD 2CP-601S 100K (50$ each). I removed the original ones and replaced with these new ones. The end result is just amazing! No errors when adjusted, no impurity in sound! Believe me or not but the sound quality improved a lot.
In fact the exchange of one attenuator took only 10min. I spent more time to stud shortening.
I suggest to everyone of you to replace original attenuator (stepped or not) to this one and you will thank me for that. And this is not all, the original lasts for 10,000 turns, this new lasts for 50 million turns! this is what I call an improvement!
Image: left original, to the right TKD 2CP-601S 100K:

Whatever. But there is a big difference between those two and easy to improve amplifier to next level.