I am thinking about using both volume controls so I control how much tubes sound from that Schiit. Crank the Schiit and lower the THX for more tube drive and vice versa. 4 driving tubes are a bit too much for headphones, most tube amps use 1-2 driving tubes.
Uh...why use a tube as an Equalizer? I mean if you really want to use a tube preamp stage as an equalizer and room DSP you might as well ditch the THX, get a tube amp, run a passive preamp signal to the tube headphone amp, roll the tubes in the headphone amp if you really want to use hardware not actually as EQ as your preferred kind of EQ.
The thing is cranking up the tube preamp isn't just "adding more tube sound." You're
cranking up the gain. That means that at some point other than the mess of having two preamps in one chain, at some point you'll be sending far more than 2V (SE) or 4V (balanced) into an amplifier's preamp's input stage designed to take 2V (SE) or 4V (balanced). That means you'll get more distortion, and that's not necessarily just the euphonic tube distortion you want to use as an EQ. You can end up just clipping the signal and wearing out the mechanical bits on the headphone drivers.
Also unless the tube works purely as a buffer like on the MF X10D and not a gainstage then running that preamp passive will bypass the tube.
However if it does work as a buffer then it'll color the sound even in passive preamp mode.
4 driving tubes are a bit too much for headphones, most tube amps use 1-2 driving tubes.
The tube in the Freya are either buffer tubes or gainstage tubes. They are
not driver tubes.
Also even if you actually used an amp with four or eight or however many driver tubes there are, it's just overkill, and as long as you don't clip the signal - the chances of which will be minimized since presumably as long as the circuit is good you can expect power to be higher, though not proportionately - it's not a problem.
But what you want to do, depending on how the Freya uses those tubes, is not just add more rectifier tubes, but add another gain stage. Regardless of whether the gain stage in the headphone amp is a tube gain stage or not, or for that matter on the preamp that serves as a hub for routing the signal, having two gain stages is a bigger problem than having an overkill powerstage. Two properly designed gain stages will add more distortion than a single high power output stage (barring something like really cheap Class H amp, like on cheap PA speaker amps; or using a subwoofer Class D amp on fullrange speakers) and can add too much gain at the first gain stage that lowering the gain on the second gain stage will not matter as far as preventing clipping goes. Your signal can still clip even if your second gain stage is so low that you have potentiometer asymmetry, so long as the gain on thefirst gain stage is high enough. In short...
you're worried about the wrong thing.
If that explanation is too hard to understand, that's all the more reason why you shouldn't do what you're planning to do. It's like getting bored on a flight because the air traffic controller ordered the aircraft to circle overhead, staging a coup on the plane and force the pilot to attempt a landing, and then end up ramming the other planes that are still being taxied into the terminal or getting towed out to the runway to take off because
there's a freaking reason why the ATC told the pilot to go into a holding flight pattern.
