I pretty much agree with most of what has already been said, and I've commented on this elsewhere, but I'll throw a few more cents in.
First off, as intended, the WD-800 is very reminiscent of the WalkAbout, both in terms of how it sounds and also how I use it. Meaning, the tone controls are laid out in similar fashion, and I tend to do approach the tone tweaking in similar fashion, with similar results. The WD-800 does bring some more options to the table, which is great, IME/IMHO. The WalkAbout does still "feel" a little different to me, but again, the WD-800 is clearly an evolution of the WalkAbout.
Also very much as intended, the Bass-800D is very reminiscent of the Bass 400/400+, again, not only in terms of how it sounds, but also how to use it. Like the WD/WalkAbout dynamic, I approach tone tweaking on the 800D very much as I would on my Bass 400, but I once again want to say how much I love the HPF on the 800D. I am also extremely impressed with how close Andy was able to get the inherent sounds of the 800D to the Bass 400/400+. It is also interesting to me how well this "simple" tone stack works, even in this world of more precise control. While I really do love and appreciate the three bands of semi-parametric mids on the WD-800/WalkAbout, it is amazing how well you can dial things in with three bands, an HPF, and a couple of switches.
So, as for how they compare, it largely is like comparing a WalkAbout to a Bass 400/400+. The WalkAbout is a bit more smooth, and you have a lot of fine-tuning options, and the Bass 400/400+ sounds a bit "bigger" and more "lively," with more of that "3D sonic texture" I really love from great tube gear. The sense of fullness, with clarity and excitement, was what initially attracted me to the Bass 400/400+ (and D-180), and Andy has done a wonderful job of capturing this in the 800D.
If you made me pick just one, right at this moment, I'd go with the 800D, and I have been a pretty big WD-800 for a while. That being said, if you told me that I could only play through a WD-800 for the rest of my life, I'd die a happy man.
