I was worried about hurting my speaker tube amp, a Yaqin MC-13S (push-pull 4x6CA7, 2x40w into 8 ohms), with headphones but after reading this thread I decided to give it a try. I started with a direct balanced cable connected to the speaker outputs and all three of the headsets used in this test have been modified with balanced cables. I used a raspberry pi (far right in pic) as the music source -> USB -> Aune T1 as DAC only -> Amp.
I listened with Modified Fostex T50RP and AKG K240 Studio cans and both sounded great. I had absolutely no issues with this setup. Max volume was around 9:30 with most music. The AKGs needed a little less volume compared to the T50RPs of course.
I then built an interface box so I could add a resistor network to match up the amp and headphone impedances. I started by only installing a 10 ohm 12 watt Mills wire wound non-inductive resistor across both speaker terminals (one resistor across each + and - terminal). I plan to add a 2 series + 2 parallel resistor network later. With this loading resistor setup the max volume on the knob didn't seem to change but the T50RPs sound incredible. I would have to say they sound better than when used with my Schiit Mjolnir. It's just a warm, wonderful sound. I was running late for an appointment but had trouble taking off the headphones to get out the door. The AKGs sound really good too but the sound quality seems comparable to the Mjolnir and Aune T1 headphone amps. The HE-500 did not like the tube amp with just the loading resistors. The tone was noticeably weak and hollow. Of the three headsets tried the T50RPs get the most out of this tube speaker amp.
I apologize for not giving a more detailed description of the sound quality but I just don't feel qualified to do so.
I love the Yaqin amp by the way. I replaced the Chinese preamp and phase-splitter tubes with Mullards but the original power tubes sound great. It's also quite a looker with beautiful chrome chassis to highlight the 8 tubes.
Shhhhh!!!! Don't tell anybody that you actually used a speaker amp to drive headphones!!!! They'll think you're crazy! Everybody knows that doing that is impossible!!! Your headphones will explode!!! And they'll sound bad besides! And your head might explode?
Well, welcome to the crazy club. As you've discovered, if you can find the right match between amp and cans, you can get amazing results driving cans with speaker amps.