Hello,
you should pay attention to one thing.
The Euforia and Elise have not been designed for KT66/88 tubes.
You must remember not to exceed the heating limit that is specified.
With the Euforia it is 7 A rather 6 A. With the Elise I don't know exactly.
This means that it is possible to use 4 KT 66/88 tubes, but it is very borderline.
It is better to use 2 or 4 Deyan adapters.
2 would be suitable for KT66/88 as well as EL34 tubes, the other 2 adapters would be for 6V6 as driver tubes.
These can also be used and offer a great sound, even better than 6SN7.
What also works are adapters 6SN7 to 12AU7, since the 12Au7 also have 20 mu amplification like 6SN7 and the market is just as full, it offers you good possibilities if you like the sound from the 12Au7.
KT88 Ps Vane Horizon are great tubes as well as Raytubes KT88 Select.
Ideally these tubes should be paired with Nos driver tubes.
Back to the heater, it is important that you do not exceed the amp limits.
There have been cases where some people have overdone it with 4 KT88 tubes and ruined the amp.
You should always be aware of this.
In an emergency, check how many amps the heater needs from the tube you want to use.