Experience of initial biasing of my amp

I own a Cayin A88T integrated tube amplifier which I bought some four months back. I sent my amp to the technician last weekend to check the biasing for the first time. To my surprise he said that one of the tubes was not biasing and was actually at zero bias. According to him, the tube was bad. Also, the left channel tubes were biasing at 0.46mv instead of the recommended 0.4mv. All the tubes are the stock chinese ones (Shuguang maybe). I luckily had a spare tube and he replaced the bad tube with this and the bias was held properly.

What I don't understand is that the amd sounded good even with the 'zero biased' tube. I would think that an amp with one of the tubes at zero bias will not operate or will sound awful. My amp sounded good???.

After the new tube being installed, the soundstage has improved with probably better detail across the frequecy range. The difference is, however, not night and day. The difference could also be due to the new NOS RCA 6sn7s that I had installed just prior to biasing.

So, what do you'll think? I don't think the technician has any motive in giving me wrong information (I was not present when he was biasing).