It should fit because all of the mounting holes are identical between the Walkabout and the Subway WD-800. Of course there will be extra space at the back. (disclaimer: I'm not familiar with all of the head cases that might have been made going back to the earlier production, nor any possible customized headcases)
I'n not a tube preamp stuck into a Subway. This is a recreation of the Walkabout concept, the basic architecture including the architecture of much of the original tube preamp but scaled differently to better meet the requests of the many comments over the ~15 year life of the Walkabout.
The reason for using the USB-A connector is purely for durability and compatibility. It better matches up with the bazillion cables and chargers in use today and likely in the future. Note that this is a 500mA charge port only, so there's no data benefits to be had by using USB-C.
This really is a different amp than the D-800 and D-800+, it's not an "upgrade" or a revision but an amp that will appeal more to those players who have gravitated to the Walkabout in the past but find the limitations unworkable.
Not sure I understand your question or comment. We are using a void free, high ply count Italian poplar plywood because it happens to be a very nice material to build speaker cabinets from. All of our Subway cabinets have used this material with no problems at all. The only real drawback is that it's quite expensive compared with other plywood materials. (Finnish Birch is also expensive as well, but is a lot heavier)
Yup, I wanted to hear from you guys what you thought was important and what you might want to play in the future. I have said it before, but TalkBass as a community is full of good players who represent the higher end of the bass market and usually do a good job of articulating what direction this market needs to go. I am ALWAYS listening to what's being said on TB, like the proverbial fly on the wall