Heh. Well that provides some valuable info. It means his battery is much smaller than the one I am using - so he can't be using the same volume control or it would never get the 10000 hour battery life required. It also means it will be push buttons or toggle switches, which I don't want to use mostly for 2 reasons: can't tell where the volume is without a display, and it loses the analog feel. I could probably shave off a couple hundredths of an inch by going with a weaker & harder to produce extrusion, but anymore than that and I'd have to start reducing the size of the battery and every mm will probably take 10 hours off the battery life. Why dont you ask how many steps and how much attenuation
Looking at diameters of 3.5mm plugs, one of the thinnest is the Neutrik at 0.35". Grado 1/4 to 1/8" adapters are 0.435", and ALO uses some that are up to 0.5" diameter. Which means any amp that is thinner than the plug is potentially going to have issues stacking with a DAP.