The absolute most important thing is that you build the amp stock first and getting it 100% working. It's far harder to debug an amp with upgrades when you don't know whether it's the upgrades or the stock build that caused the issue.
I would put the DC filament in first, because it's far easier than the other two upgrades. I would do the SR driver next, then the attenuator last, mostly because you'll be well warmed up by then and you want to do your best work on that attenuator.
