I went from a Jeff Rowland Model 2, to a Pass Labs XA30.5, to a Primaluna Evo 300 Power amp, currently driving a pair of Tannoy Legacy Ardens (93db sensitivity vs the SF's 91db).
I could happily live with any of these amps in my system. I don't think an Evo 400 would be a step backwards provided the drop from 400w to 70w isn't a concern for you. However, it's not a great time to be a tube swapper given the current pricing of tubes. I find the Gold Liion KT-88s and Tung-Sol KT-150s provide the best (and similar) sound in my system. A set of 8 KT-150s could set you back $1500 and they're not the most reliable tube out there. KT-88s would be a good place to start if the stock EL-34s aren't doing it for you.