The KT-90 is a very highly sought after tube. In particular the type III look for that variation mark wherever they put it. It is not universally admired but demand is desperatley high. It is a Sturm und Drang tube that my English tube guru called timebombs. They have the ability to put out big numbers for wattage.
The 6CA7 is also valuable because it is the true fatboy type. That tube is a tetrode whereas EL34s are often described as one tube. This is untrue they do sub for each other but the EL34 is a "beam" pentode. In my amps there is an astonishing difference in the sound. With true 6CA7s which are available from EH they sing gloriously, with great dynamics, slam and authority across the bandwidth, EL34s are a mere 98lb weakling by comparison. This is only in my circuit as far as I know and not a general attribute. The only other true tetrode made these days is the reissue KT-77 which I liked but melted one of my tube sockets and are known for shorting and unreliabilty.
The 6CA7 is also valuable because it is the true fatboy type. That tube is a tetrode whereas EL34s are often described as one tube. This is untrue they do sub for each other but the EL34 is a "beam" pentode. In my amps there is an astonishing difference in the sound. With true 6CA7s which are available from EH they sing gloriously, with great dynamics, slam and authority across the bandwidth, EL34s are a mere 98lb weakling by comparison. This is only in my circuit as far as I know and not a general attribute. The only other true tetrode made these days is the reissue KT-77 which I liked but melted one of my tube sockets and are known for shorting and unreliabilty.