Just a general compare/contrast? Or is there something specific you want me to dive into?
One thing to note already is clarity. I love the Walkabout for what it does. (I have done many a gig with that stack. Kills every time.) It hangs with any amp as far as warmth goes. I don't buy into it being so "loose" on the bottom end. But, then again, I use a Broughton Always On in front of it, so there's that. But it will get punchy for days.
The Walkabout and Boogie channels share some common ground, although different. They both can do old-school gooey goodness.
However, the Subway channel runs laps around the Walkabout if you're looking for clean, articulate, straightforward bass tone. It's by no means sterile. It still has a tube front end. It has lots of character. But it can get into crystal clear territories where the Walkabout gets left behind.
Short version: Although different, the Boogie Channel and Walkabout are "close enough for rock n roll" as far as sonic territory they cover. The sound of the Subway channel is simply not available to the Walkabout. And I adore the Walkabout, so that's not a knock on it.