cuiter23
Headphoneus Supremus
I'm glad to be of service. The Vali is a very dynamic sounding headphone amp that will do well with the K7XX. Since you aren't going to be using more sensitive headphones with it or any pre-outs then the Vali should be right. The Magni will drive the K7XX but it won't give it that extra bit of dynamicness that the Vali will. Solid state amps in that price range tend to sound dull compared to tubes. The Vali will give the K7XX that extra bit of treble extension to open it up, and will bring out that mid bass impact. The K7XX will improve with better sounding equipment you plug it into. You should be set with it for a while, even if you decide to upgrade systems.
As a general rule, AKGs sound better with punchy sounding entry level hybrid solid state/tube amps like the Vali or Hifiman EF2A (not as good sounding as the Vali but has swappable tubes), than with similarly priced all-solid state amps like the Objective 2 and Magni. I don't like Fiio amps as they tend to sound dull and soft. The Vali and Modi are a great little entry level dual unit headphone system to have, especially for the price of 230 dollars or so. And they have a tiny footprint on the desk. They are also cheaper than buying an Objective 2 and ODAC, which will run you around 300 dollars for the two. As sound goes I think they're as good as a desktop system is going to get before you start spending over 500 dollars for an amp and over 300 dollars for a higher level DAC. They will also make a great workplace office desktop combination. The K7XX, Vali and Modi are likely as good a desktop open back headphone, amp and DAC system as it's going to get for under 500 dollars, especially as you don't like IEMs.
As a general rule, I prefer to pair cold or neutral headphones with warmer tubes. But for already warm and lush sounding headphones I will prefer solid state to punch through the warmness with added detail and clarity.
So yes, I would pair the K7XX with a tube. I cant say much about the vali though.