IME EF86's will be happy and less troublesome with some anti-vibration mounting of the valve socket - maybe consider even a dual anti-vib setup.
EF86's have lots of gain and will give you a huge signal, (in the AC15 it feeds the PI directly) not sure how you will benefit from that before the echo.
What HT?
With one ECC83 you can get the grid leak biased pre as well as a output mixer/buffer as per the original Meazzi. The correct HT for that would be around 170/130v respectlively for the buffer/pre.
A grid leak bias configured ECC83 works better than a EF86 for a echo pre IMO - YMMV
Erik