I own HD600s, and I think the Veritas would be a good choice if you like the Senn sound. The RAAL tweeters have a similar sound to the top end of the Senns- the highs are just there, as part of the whole; they do not call attention to themselves- no stridency or emphasis at all. The Scanspeak woofer is just as amazingly natural- very clean and detailed, with all the elements in balance. My 600s were a standard for me, and both of my Selahs (SSR and Tempesta) are in the small group of speakers I've owned that offer sound that is comparable and similar.
Ported vs. sealed- my SSRs are sealed, the Tempestas ported. If I was to order them again, I would go ported with both. The cabinets are larger, but the bottom end extension on the ported cabinets is much greater. I use my Tempestas with a sub with no difficulty at all. I've experimented with port stuffing, but I like the sound better without. YMMV, of course. The sub needs to be very clean, with a sharp rolloff on top, to avoid messing up the wonderful midbass. I think the greater extension of the ported does give greater long term flexibility in their use. The Scanspeak woofer in the ported cabinet goes low enough that a sub is purely optional for any type of music - even pipe organ!
Rick's speakers are terrific choices - one rarely (ever?) reads about anyone that is not happy with their purchases. Not "stepping stone" speakers; they take you all the way to the music.