You have to be careful using the inexpensive tube “buffer” preamps. Some are just straight wires input to output. The tubes only see filament voltage. I have one; only when I pulled off the covers, traced the circuit board, did I find that there was no “tube buffer”.
Another one I have uses a unity gain opamp with only filament voltage to the tubes.
Three times is the charm….The third one I bought actually uses the tubes for unity gain.
And….The whole idea is to add tube harmonics to the sound! So yeah, they are distortion machines!! LOL!!!!