Shag,
I was reccomended the same thing. Let me know your thoughts. Personally I didn't like them at all, made the sound muffled in mine and the overdrive is harsh. I swapped the ECC83 back to the Sovtek which helped, but I accidentally broke the EL84.
I'm putting in an Electro Harmonix EL84 and either a Mullard 12AT7 or a Brimer 12AU7 in the next few days. I want a lot less gain. The amp does cleans so well I'm going to drive it with pedals :).