The tubes in the Walkabout are only in the preamp. They last for many years and if they fail, are cheap and easy for anyone who can use a screwdriver to replace.
I currently own a D-800, a Walkabout combo and another Walkabout head for use with other cabs, and I have owned a TH head as well. Keep in mind the cab plays as much of a role in tone as the head. All your options are good. My fave is the Walkabout, but if I could only have one head, I'd pair it with a Subway 115 cab rather than have it in the 12" combo.
This mirrors my experience with tubes and specifically with the Walkabout and other Mesa bass amps. I can't recall a preamp tube ever failing on me in any head and have only replaced them because I figured after 5-10 years of heavy use my luck might be running out. I wouldn't either buy or not buy an amp simply because it has a tube or tubes in the preamp section - let your ears tell you if it's the right head for you.
I too have a later version of the Walkabout head itself as well as one of the early Walkabout Scout 112 combos. The Walkabout is something special in my experience, but it may not be everyone's cup of tea. I like its smooth, full, rich tone that sort of blooms into the room rather than arriving with a bang. The Walkabout head with the Bergantino CN212 cab is very sweet sounding with a lot of substantial presence - more so than the Walkabout Scout 112 combo while still similar in overall tone. The Walkabout is one head I'll never give up.
I also own the TH500 (which I play mostly with the Bergantino CN212) and a Mesa D-800 (which I play mostly with the Subway 112 and 115 together). The TH is not as versatile in the sounds it can get as the D-800, at least to my ear, but is a great amp especially when you're not looking for a lot of high end or a hard cutting sound. It is rich in mids and feels a lot like you might be playing an amp with tubes even though it's solid state. The TH slots in on the Walkabout end of the scale for me with the Mesa Carbines and GK MB's holding down the other end of the spectrum.
It's great amp from an excellent company and I have no plans to get rid of mine.
My current favorite, for a lot of reasons, is the D-800. I feel like it can fill most of the tonal territory between the Walkabout/TH end of the spectrum and what characterizes the Carbines and GK heads. The fact that the head and cabs were designed to be complementary and work as a system where you can mix and match 112 and 115 cabs is also noteworthy. The Subway series is exceptionally well thought out in both design and execution by one of the industry's top engineers and it shows in the final product. Mesa's reputation for quality and customer service is well known to guitarists, but I would say it is equally legendary among bass players who know the brand well. I'm not sure it can sound exactly like a TH, but in a busy band mix the D-800 can give me a lot of what I like about the TH yet still slot in very distinctly. If I had to keep just one head, it would be the D-800 (with Subway speakers) because of its ability to cover the majority of the tonal spectrum I work in.
My Ampeg experience is from decades ago, so it's not relevant here, and I've not owned or played any Genzler amps. The bottom line for me is that your list has different flavors of good that on any given day might be someone's first choice for arguably good reasons. Best of luck with your search.
