Tube Characteristics

I have recently been advised to try the Russian 6CA7 as a replacement for the EL 34's in my Manley 250's. Anyone familiar with this tube. If so how do they sound/differ from the EL 34?
They are perfectly interchangeable. I tried some Electro Harmonix 6CA7 in my CJ MV55. Compared to Electro Harmonix EL34, they had more powerful bass but less delicate highs. Which is better is a matter of personal taste and system setup. Try them and see what you think - it is the only way of getting a real answer.