socketing the resistors, for my money, doesn't make sense on the PPA. The connection method is simply sticking the leads of the good quality resistors into sockets designed to take chips, no solder or anything. Not the best connection method, and to me negates the use of high quality resistors such as the Vishay-Dales your builder is likely going to use. Also, unless you know what you're doing, you'll have to know which resistors you need to change and to which values, although it's not terribly complicated. However, to balance the impedance into the opamp (I know, this is prolly a little over your head, but I'm sure you can learn =) ), you will need to change other resistors, and then more resistors after that, so isn't really that simple.
If you are set on a PPA, a gain of around 5 will do almost all headphones quite nicely; on low impedance (low-Z) phones, it will mean you're restricted a bit on the range of sensible volume control, and it may not have the juice to drive really hard loads such as the AKG-1000, although they really need more than a dynamic amp anyway.
Just by means of comparison, I use Sennheiser HD600s (high-Z), which are a very common headphone around here, and every amp I've built for myself has been set to a gain of just over 5, and I never get past 12 o'clock on my dial before it hurts, although I do listen quite quietly.
hope that was helpful
g