EL 34s for Cayin A 50T Integrated Amp


I am thinking of changing out the stock tubes (The EL34s) in my Cayin tube amplifier, but don't know what brands to look for. I mainly play rock and I am looking to improve my base and possibly the level of detail as well. Does anybody have any suggestions?

Thanks for your help!
marntz4me
I saw a quad of the EL34 sveltlana's on ebay. They were about $59? One last question, does anybody no about EL34 Mullards? They are selling them on Ebay too.
I'm happy with the EH 6CA7s. They replaced the EH EL34s which is quite good too.