I used to use a `Mesa Boogie V-Twin valve pedal. The clean sound was out of this world. To be honest I was never that keen on the overdriven/distorted sound, though that's a matter of personal taste. It's certainly very highly thought of. Here's a link:
http://cgi.ebay.com/MESA-BOOGIE-V-TWIN-TUBE-PRE-AMP-GUITAR-PEDAL-OVERDRIVE_W0QQitemZ320472337097QQcmdZViewItemQQptZGuitar_Accessories?hash=item4a9da3cac9#ht_500wt_977
As you can see, they don't come cheap. I've never heard a digital emulation, good as they are, that comes close to a real valve sound. Even for clean sounds, and if you really want to sound loud and dirty, or loud and soaring, you just have to play loud through a loud amp. It's not just the valves - it's the way the speakers respond and how your guitar starts to self-oscillate with all that volume. IMHO
Go into a music shop and try a good make of amp with a 2x12 cab, even at a fairly discreet volume. Then go home and try GuitarAmpPro. It's really good, and will sit nicely in a recording, but if you're not content with that you'll have to do things the old-fashioned way.