These valves were perfect for the application I wanted. I used them in a 20 watt Jet City JCA20HV to beef up the headroom of this amp further (along with a NOS 5751 pre-amp valve) and make the amp far more reliable for gigging. I feel these have enhanced my tone and have added a hi-fi like character with deep, extended lows and a clear, rounded top-end.
Because they have a a much longer life than standard el84's I have biased my amp slightly hotter to really assess what these sound like pushed. They still manage to retain clarity and have a much smoother character than the standard valves - think of a mid point between an EL84 and EL34 with more natural compression. If you play a lot of clean, jangly stuff and have Vox style amp, these will really open up the power section and you'll love them! If you have something more akin to a Marshall that's very crunchy and growly, then perhaps your power section won't be working hard enough for you with these.
Of course with anything, YMMV - Get yourself a set and experiment!