The joys of getting interrupted by life when typing.
50k to 100k POTENTIOMETER resistor. The idea is to quickly change the response until he finds something that works. I don't know about tube output stages for headphones. On a guitar amp you just tap off the transformer. /shrug
Going series or parallel gives different characteristics. I have had good luck going parallel IE tip/ring to ground when I last tried to drive headphones with a 25watt car stereo when I used to tinker with stuff like that.
50K or 100K is, like I said, easily 250 to 500 TIMES too high. If you want to play with series resistance, you'll need to be between 0-200 ohms. Terminating any amp designed to drive speakers with even hundreds of ohms is completely pointless as far as it seeing a load.
If you connect the pot so the resistive element is across the amp outputs, then take the headphone output from wiper to ground, that's fine, but, again, your pot values are 100s of times too high. You need to be in the 100 - 200 ohm range. In the range where it will actually sort of work, you've blown away whatever damping factor you had, and again, you won't load the amp with anything like it was designed for. Tube amps are nothing like car stereos.
Sorry to be blunt, but the whole idea makes no sense at all, unless you must want to get some sound, any sound, out of your headphones and don't care about levels, noise, and amps and pots burning up.
We live in a world of fantastic headphone amps for $129, and decent ones well under $100. There are 20wpc stereo amps, good ones, for $30, and with any of that you'll have none of these issues. If you want a tube to drive your headphones, invest in a tube headphone amp. The reality is, tubes aren't great at all. Neither are transistors or ICs. It's the circuits they're in that makes them great. A mis-applied circuit, or a badly designed one will always be far worse than a cheap and simple, but properly designed and applied one of any topology.
Just to make sure you know I'm not just talking out of class here, I have actually connected headphones to a 35 watt tube (6L6, class AB push-pull) PA amp. Yes, I got sound, no it wasn't good, and I did burn parts up before I realized my approach was wrong. That was a hand-full of decades ago, but when I turned up to volume, I'll never forget the smell of burning pot...er...potentiometer...yeah, that's what I meant to say. No, really.