Yes, I think a monoblock design with 4 chassis wouldn't work that well for a headphone amp. Whichever config I end up with, I'm going to keep it to a single PSU chassis and single amp chassis, but try to lay everything out so that upgrading later on won't be that bad, if I decide to go that route.
I've been playing around with panel designs and enclosure layouts just to get some idea of how feasible it would be, and I think I have something that would work... using Neutrik D-series jacks for the RCA inputs and headphone outputs, that way if I decide to go balanced, replacing those with the same series XLR jacks will be trivial.
The Ecomate power connectors come in a 6-contact version, so I'll plan on using that from the start, with a 6-conductor umbilical, and that keeps me covered later as well.
As far as gain goes, from what I've read, I think I'd need to change if I upgrade. I'm going to be ordering D7000s in the next few weeks, which would need lower gain, but will also be using the amp with 250 ohm DT-990s. I've settled on a gain of 5 for the unbalanced config, but if I go balanced, I'm thinking I'd need to drop that to a gain of 2 per board to account for doubling. Still, though, as long as I'm careful with the boards, swapping out the gain parts shouldn't be too bad after the fact.
I'm still deciding on which way to go for volume control. I've got some email out to TKD about their stepped attenuator - according to their data sheet, it doesn't come in a 50K version, and it's rated at "0-15KHz frequency" - I wasn't sure if that had any effect on using it for this, though from everything else I've read, it seems to be a good choice. If they don't have it in 50K, though, I need to decide on an alternate. They've got 10K, 20K and 100K listed in the data sheet.
Another potential downside - no quad version of the TKD. So if I upgrade, it also means a change of volume control, so I need to think about that for the layout as well.
Still time for decisions, though.. I'm not rushing into anything... I want to make sure I plan this out well before I start.
-Mike