Think of tube head amps as a different musical instrument, when I want to listen to jazzy bluesy stuff, they really scratch that itch.
When I plug into my tubes I use my Beyerdynamic Amiron's. These are 250 Ohms and the TA-66 tubes prefer >66 Ohms up to 600 Ohms. I believe the Sennheiser's HD6XX with high Ohms will work well with tubes.
My planars have reasonable sensitivity and don't require driving hard but low Ohms, may not be best for tubes, I have these plugged into my SingXer SA1 class A headamp, and sound fab on solid state.
If you like vocals, the tubes will enrich every sinew of the sound, particularly females and string instruments really come across well.
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