Really none. Every amp sounds a bit different from each other, and the Walkabout's warm, tubey, midrange complexity is among the most unique voicing out there. Even the MPulse600 sounds little like the Walkabout (much bigger low end, cleaner, smoother mids).
Small heads that at least can be considered 'tubey and organic' sounding, if in a different way from the Walkabout, are IMO the Aguilar TH350/500, the Genz Streamliner 600/900, and the Orange Terror500/1000.
I would think, if you were on a budget, the Aguilar TH350 would match the Walkabout's output into 4ohms, be VERY affordable, especially if purchased during one of Musician's Friend's 10% or 15% off sales. Also, the good news about buying from Musician's Friend (assuming you are in the US) is that you can return within 30 days, no questions asked, and only be out a bit of shipping.
The TH500/350 is voiced very 'organically'... big low end, VERY complex present midrange, and a relaxed 'non sizzly' top end. Again, doesn't sound like the Walkabout (nothing does), but it is in that tone universe I call 'vintage meets modern'. Nicely featured also, and super reliability/quality IMO.
Last edited by KJung : 01-12-2013 at 08:01 AM.