[Honeypot]

Ultimately depends on your taste.
What I mean, is there are a lot of great options and none of them will be "the one" for everybody.
For instance, tungsram 12au7 with it's adapter it's a joy. The Amperex bugle boy too. The tungsram E88CC it's another cheaper option which is also very enjoyable. Mullard another superb option.
How much do you want to spend?
What sound characteristics are tou trying to achieve?

 

[Honeypot]

For those people with the echo, I have recently started using it as a preamp.
To give some context, since I got the zmf caldera I started using more and more echo as an amp, seeking how warm and holographic Caldera could go. Which ultimately lead me to start tube rolling and experimenting how this little amp can perform. But due to output impedance, many people stated that the caldera as a low impedance planar could work better with solid state amps or OTC tube amps. That brought my curiosity up and ended up purchasing the zmf atrium open (still waiting for it).
In the meanwhile and trying to get the most of the caldera (and as I am currently out of budget because of the tube rolling and the AO purchase), thought to myself, what if I pair the zmf with my ss amp (flux Fa-12) with the echo as a preamp (with the new tubes), that way I wouldn't be conditioned by the high impedance of the otl echo and still get the flavour of the tubes.
Well. Surprisingly surprised.
I have no words. Just try.

Let me know what you think.

And excuse me the wall of text without proper separation and punctuation.
But I am writing this while going to pick up my kids from school

Have a nice day everyone!!