I am planning to get my 1st tube amp for specifically headphones, primarily my HD 600 and ZMF Aeolus. Thing is I don't know which. I don't what to look for in a tube amp. It's different from solid states. In solid states I look at how much power they give, output impedance, any gain switches but what is it to look for in Tube amp? I just wanna look for a warm,dark tube amp. Also must I buy my own tubes or are the stock ones good enough? Lastly can someone explain to me why I am not recommended to pair tube amps with planar headphones?