Like Gluegun said, these sound totally different from each other. I dont know about everyone else, but i own both and let me give you a quick review:
Sony V6's/7506's:
Tight, deep bass. Clear highs, a wee bit on the bright side. Sound amazing on my MD player and very good on my discman and receiver. BIG, your ears sit inside the cups, especially if you replace them with Beyer pads. My best headphones so far, i love these things, they are closed by the way. They sounded hella bright for the first 2 weeks i owned them.
Senn 495's:
Without an amp: they sound ok on my MD player, doesnt drive them loud enough, ditto for my discman and receiver (for the most part).
With a cmoy amp: a little better, um, these sorta sound like the Grado 60's, but nowhere near as bright. Also, i have found they sound (with Guitar-driven music) to be dull, lifeless, flat, neutral, a little wimpy in the bass department and grainy. Not to say these are analytical, but music that has been poorly recorded will just awful (such as linkin park's hybrid theory) on this cans, thats what i've noticed so far. I've burned these in for a few days and the sound should improve. They do sound better with quieter music (such as piano and jazz...haven't tried any classical music yet). In other words, these are probably better suited for softer music. Definitely not Rock headphones.
Besides Link Park, i used Lenny Kravitz's '5,' Paula Cole's 'This Fire,' 'Happy Anniversary Charlie Brown,' by various jazz artists and Verve Pipe 'Villians' These all sound fine on my 7506's but only the charlie brown cd and Paula Cole sounded ok with the 495's.
Personally i like my Grado's better (better clarity, albeit too bright for extended listening), and especially the Sony's (ahhh...my fav's!). I'd also like to remind everyone: I've only owned the 495's for a week so i will give them time to break in. Best of luck!
George