If I was putting a DAC in the same case I'd definitely not use the BOM case or even the longer Hammond case. The longer case is tight enough as it is. However, it depends on I/O components that you choose to use though. If you were using 1/8" jacks and a RK097 on the front instead of a RK27 and 1/4" jack then that would give you more room. Also, if you wanted RCAs that would also cramp things a bit on the back. The best way to do this is go with 1/8" jacks on the front along with RK097. Then skip out on the fuse and RCA jacks on the back. That way you just have a power switch, USB for your DAC, air wired DC jack, and Heater switch. Are you going with 4 heater voltages? That NKK switch takes up a bit of space as well. Overall, if you want to accomplish a lot I would take a look at that case that cfcubed is using for his Fancy CTH. It looks to have space on the sides of the CTH board that would allow for a DAC.
With regards to your worries about the PCB and it needing more support. It fits snuggly in the BOM case. The tube socket is fine too. The only thing that I'm thinking of doing is getting one of those socket savers like cfcubed uses. I hear it's a PITA to change out a tube socket if one goes bad.
Perhaps, Forte can let us know if he thinks a socket saver is required for tons of tube rolling.
I've got a whole list of tubes and thus far I'm liking a bunch of them. Siemens ECC802S, RCA 6CG7 Clear Top, Raytheon 6GU7 Japan, Telefunken ECC82 Smooth Plates, Sylvania Gold Brand 5814A gold pins to be specific. There are tons of others that are good as well. Tonight, I'm auditioning 4 different 4BZ7 tubes. So far I'm liking them the least. They seem to be a little shrill. And the bass seems a tad bit muddy. Perhaps, this is partly due to using my KSC-75s with this amp. I'll switch over to my K701s for comparison.