Anything that will work at a real KT66's load impedance and voltage levels should be fine in this amp. They're not being stressed at all. However, you probably won't get tone that's quite so good with the "re-bottled" 6L6s, so I would tend to recommend real KT66s for tone.
You may also recall these comments from Richard Johnson:
rjgtr wrote:
I like either the Shuguang or JJ KT-66s. I find the Shuguangs like to be biased hot.... I think I used about 75%. They just growl a little better there to me. I read about it in a review of them somewhere and it works.... The JJs are a little purer sounding.