Can 6AJ8's be safely substituted for 6KA8's?

+1, As noted above, NO.   6AJ8 tubes cannot be safely substituted for 6KA8 tubes. The two tubes have different pinouts and internal structures: the 6AJ8 is a triode-heptode, while the 6KA8 is a high-mu triode and sharp-cutoff pentode.

Using a 6AJ8 in place of a 6KA8 could result in improper operation or even damage to your equipment. Please be careful.  

Thanks to everyone for clearing this up for me. I really appreciate it.

@oddiofyl +1

 Lord! I just got a flashback of spending hours with my dad on a Hickock tv10 testing tubes. Ahhh, those were the days.😁

A lot of good equipment has been hurt by the wrong tube.  I always use exact spec when possible.    

Brent Jessee' website is a valuable resource also.   Lots of info there.  When i think about it that's where I'd start .  I go there occasionally,  I have gear with obsolete tubes