Do new tubes undergo break-in like other devices?

I just put a new pair of Mullard Ecc34 (should be 6sn7s) tubes in the r/l inputs on my BAT VK60. These are the 2 most importants positions for BAT amps.

Part of it sounds wonderful...back of the soundstage, instrument separation, dynamics, articulation in the bass. Oh, and much queiter.

However, I don't think they have as much gain, I have to turn up the volume some on the preamp, and the bass is much lower vs the other parts. Kind of like a new power cord...needs time to open up. I can deal with the gain, not sure about the bass. This may just be a system matching thing, works great for some, not for me.

Is it break-in related?