Mark II Tube Questions 6L6 STR 420

[Woodlumn]

Hi all, I have a Mark II combo. Last night I could smell some burning smell, more than the usual pleasant odor it gives off when warm. Then there was some loud bursts of crackling from the speaker so I went to standby then powered off.

The two 6L6 tubes have black/silver around the base, a burned look to them. I'm guessing they are the culprit?

Looks like it's tough to find a matched pair of 6L6 STR 420 tubes. Are the Shuguang 6L6GC tubes as good? Should I be considering another type of tube?

Here's a picture, do these look burned out?

Thanks!