Tig:
You probably need to do what I had to do, print the article on tangentsoft and read it about 10 times!
You have two different loop gains, the inner loop is the two resistors associated with the opamp, R5 and R6. The outer loop is R3 and R4. R3 and R4 set the overall gain. There are two more "restrictions, the outer loop values should be lower than the inner loop values, so R5 should be 5-10 times higher than R3 (I reversed Tangent's wording). And you want a pretty high gain on the inner loop (200 to 400 believe it or not), to limit the bandwidth of the opamp and make it more stable (and there are some posts somewhere that this also substantially decreases distortion).
The gain calculator gives you values for either of the loops, but not both together.
So take a simple example rounding your values to real standard values:
R4 - 10K
R3 - 1K
results in a gain of 11 (this is the standard CMOY configuration)
So now you want R5 5-10 times higher than R3, so:
R5 - 5K (actually 4.99K)
Now to get a gain of 200, you would need R6 to be 1M
On top of all this, you said you have several different sets of cans, generally speaking, you can set a lower overall gain (2 or 3) for Grados, which are 32 ohms, and if your AKG's or Senns are 600 ohm cans, you should probably go with the standard gain of 11. Don't really listen to me on this, I don't have any 600 ohm cans (but I would like some 580's or 600's someday). So you'd better build two!! Sorry about your wallet!
Actually, just do what JMT said, order sets of resistors for both gains, and use sockets on the meta42 board, and see if you can reach a decent compromise, maybe your aforementioned gain of 5 would be okay.
yeah, buy lots of resistors, that's the ticket!!
Good luck
da_burl