As you've found, there is lots of conflicting advice out there. Here is one of the more negative opinions:
http://www.audioasylum.com/audio/tubes/messages/16/168747.html
I note in my tube manual that the 6DJ8 is listed as having a nominal u (amplification factor) of 33, while the 7ES8 is listed here as having a nominal u of 65. Perhaps operating the 7ES8 with only 6.3 volts on its filament, as opposed to the specified 7.6 volts, would bring those figures closer together. But I would not expect anything better than a rough approximation, which at best would be good enough sonically only in some circuit applications.
Regards,
-- Al
http://www.audioasylum.com/audio/tubes/messages/16/168747.html
I note in my tube manual that the 6DJ8 is listed as having a nominal u (amplification factor) of 33, while the 7ES8 is listed here as having a nominal u of 65. Perhaps operating the 7ES8 with only 6.3 volts on its filament, as opposed to the specified 7.6 volts, would bring those figures closer together. But I would not expect anything better than a rough approximation, which at best would be good enough sonically only in some circuit applications.
Regards,
-- Al