Electro-Harmonix 6550 Matched Quad
6550EH is a little brother to KT88EH. It handles a little less power than KT88EH, but otherwise the two are identical, with good amount of high and bottom end energy.
This makes 6550EH quite a bargain if your amp can take either 6550 or KT88.
Price is for a matched quad of Electro-Harmonix 6550EH.