My initial impressions of the V550 Pro after some running in...
It has a ton of power and voltage so it does run both the LCD-XC and the HD800 with zero problems and i doubt there is a set of dynamic driver or planar magnetic it can't run however, i prefer the HD800 on the Vio and LCD-XC on the Benchmark. The Benchmark has more slam and impact at lower volume, the Vio has to be cranked up to get near similar amounts. The Vio is on the warm side of neutral and that's great for the HD800 but not so much for the XC. Lovely preout though, really nice presentation and great soundstage, a touch warm through the Harbeths but still lovely. I think i prefer the Vio preout because it sounds so good.
Both amps deliver fabulous soundstage and imaging however, the Benchmark is a bit more detailed, and voices are are slightly forward which i like compared to the Vio slightly recessed voices that sometimes disappear into the music. It could be the Vio just needs more time to run in but it's been running almost continously since it arrived.
In an ideal world and if money was no object i'd have both, one for the XC and the other for the 800. I'm very much still in two minds about which one i'll buy, i need more time with the Vio before i pull the trigger on either.
My Mrs prefers the Vio for the warmer presentation, i'm a detail lover so we'll see what happens. To be honest i am slightly leaning towards the Benchmark at the moment but i really would like to give the Vio a chance to change my mind so watch this space.
Someone wrote the Vio is headphone agnostic, that's not what i'm hearing.