I am looking to build an audio amp soon for the tweeters in my four way active speaker system.

The requirements are
stereo amp or dual monoblocks
balanced input
15-20 watts output
.5-1 volt sensitivity
ability to drive a 6 ohm load

I figure maybe two 6V6 or 6L6 per channel and one 12AX7 per channel would give enough gain and power output.

Main thing I am wondering is how would I apply negative feedback if I need it?

I plan on using Edcor transformers as they are reasonably priced.

I was also wondering what TV tubes I could use perhaps with a triode + power pentode in one envelope that would do the same thing?

Could look into the 11MS8, 11BM8 or 10GB8 triode-pentode tubes (or other heater versions of the same)...a pair of them will give you one triode section for a phase inverter, one for a pre-amp, and two power pentode outputs, should be good for 8-16W of power. Run 4 of 'em in parallel push-pull if you want.

I would run the amp from the balanced output of the crossover so would not need phase inverter.

Here is something pretty close to what you want. Simply rewire the phase splitter into a differential amplifier. The global feedback can probably be eliminated without problem if an "ultralinear" setup is used. If the tetrode setup is used, local feedback could be added. It might work to take global feedback from the 16 ohm and the common terminals if the four ohm tap was grounded, but I suspect having a speaker attached might mess up the balance.

That looks simple enough. I would only need to remove the 470K and 510K resistors in the 12AX7 plate circuit, the .05 uF cap in the 12AX7 grid circuit the 200 ohm resistor in the 12AX7 cathode circuit and the 1500 ohm & 680MMF feedback components.

I will look at doing that if the amp I am getting doesn't fit the bill for what I need. Of course I could always mod the amp I am getting which is push pull 6BQ5 stereo (not sure who made it though) to have a balanced input via 1/4" TRS jacks.