It's possible to use EL34, KT77 and KT66. If I remember correctly a hole must be punched in the chassis top to clear the KT66.
Best (most accurate) sound is probably with the EL34, best of those in my experience is old, genuine Mullard.
The KT77 Genelex is excellent and what I ran in my Aesthetix but there‘s a Lot of difference in price. For most folks the Mullaerd EL34 is by far the best value and a great overall match for Aesthetix sound.
The Genelex (MO Valve) KT66 is a more weighty and warmer sound so depending on your system, room acoustics and other tubes in the Aesthetix there are lots of variables.
By the way, the chassis punch to open the Aesthetix lid is expensive for a one time use. I might be able to find specs on the one I used. Other option is leave lid off but lethal voltages reside in the Aesthetix power supply, as most high end tube electronics :-).