Speaking of contacting the manufacturers, I got a response back from Jack of Woo Audio. I want to thank Jack for getting back to me in a timely manner and taking time to answer my questions. Anyways, here's his response to my questions regarding WA6:

1) What's the proper way to turn on WA6 with headphones? I ask this because I've read some stuff about it being harmful to the tube amp to turn it on without headphones in? Is this true? So far, I've turned on the WA6 without any headphones inserted (when I turn it on with headphones in, it gives the loud startup tube hum then goes away after about 5 seconds; I wasn't sure if this is harmful for headphones or not, so just to be on th safe side I unplugged it from there on), but plugged it in about 20 seconds later when the tube startup is done. So should I turn it on with headphones in or out (or does it even matter)? Just in case you may be concerned, yes, I always make sure the volume is down to zero when I turn it on or off.

J: It is recommended to switch on the amp with a headphone on. This would avoid the amp being open circuit - not a good thing for any tube amps. You will not harm the amp at all if you leave an open circuit for a short period of time. The loud noise/abnormal sound is normal the tubes are warming up to get into the working state. A proper way to use the amp is 1) plug in your headphone, 2) mute the volume or stop your source 3) switch on the amp and let it warm up for at least 10 seconds, and that's all.

2) I know the length for tube warmup for amp varies, but what I was wondering was; is it actually harmful for the amp/tube if I crank up the music right away without warming it? I realize the sound won't be optimal in the beginning but just wondering if this is actually a harmful practice to not warm it.

J: No harm at all.

3) If I'm understanding correctly, the headphone should be plugged in at all times when warming up the amp, or else it's harmful. Is this correct?

J: Same as 1. So again, it is a good practice when using a tube amp.

4) Is it safe or unsafe to flick the impedence switch in the back while the amp is on/while the music is playing? I always thought no, so I've been turning it off every time before I do so, but wanted to make sure.

J: You can change the impedance setting while the amp is on and music is playing.

So I guess the way Jack recommends it is that I keep the headphones in the jack when I am turning it on, but apparently you won't harm your amp if you leave it as open circuit (without headphones in) for a short term.