Bad Tube?

Hello,

I am new to tubes so forgive this question if it seems silly.  Last month I picked up a pre-owned Muzishare x7 integrated tube amp.  I enjoy the sound very much, and it is functioning fine, but I read that upgrading the stock tubes could improve the amp even more.  I purchased a matched quad of Shuguang kt88_Z black treasures power tubes  as well as a pair of electro-harmonix 12ux7's and a Gold Lion 12ax7 (I haven't touched the rectifier tube).  I dropped my bias to zero on all the power tubes, replaced the tubes, and re-biased  the power tubes.  They sound much better than the originals immediately.  Still detailed but weightier, if that makes sense.  My problem is, after 1/2 hr, I noticed audible static from my right channel.  Checking the V4 bias meter showed that the voltage was not settling at 40 (needle is fluctuating between 30 and 60).  I zeroed out the bias on all the tubes again and after letting everything cool off for a bit, swapped my V4 tube into the V1 position (left channel).  Starting the tube up and biasing them all again, I now hear static in the left channel and the V1 bias meter is not stable.  I have only 1/2 hr on the tubes.   Is that tube bad?  or will it settle in? Or did I do something wrong?

Thanks so much for helping a new tube enthusiast out!

Ed