Not at all. Maybe my explanation are not clear...
Please share an example of how I would go about this?
Ok. Let's say your multitrack have max output of +23.5dbu.
The circuit max input voltage is 13.5dbu and you don't want multiple step on your attenuator, only one position.
So overall you'll need 10db attenuation. If you use the sparkfun card you'll have -6db de facto. So you need to make an attenuator of -4db. You want something like 10k impedance for the attenuator (same as pot) and an L pad (two resistors, simple):
http://www.sengpielaudio.com/calculator-Lpad.htm
As a result with something like a r1 of 3.7k and r2 of 17k ( R number referenced to the sengspielaudio calculator) it should be in ballpark.
Your input is done: -6 + -4 : -10db.
Now about your output: you'll need +10db gain overall. As is, the circuit is at unity gain when you bypass r48/C5 for u6 and as i already stated there is +4db with value given around u5a and b.
So to have +10db the values around u5a/b have to be modified for a gain of +8db (the already present +4db + the db needed to reach +10db (minus the +6db gain in the 1646), so +4db, hence overall gain around u5a/b: +8db).
So if you keep r7 20k and make r6 13k: 20log(1+20k/13k)= +8.09db.
Done.
Repeat for the number of step you want... in case you want other step with different attenuation and compensation...
Anyway, try what abbey road said.
My idea is for educational purpose and if you need multiple headroom situation.
(and you'll have one and only control for the whole circuit with a deck for input and another 2 for output compensation).
...well hmmm, in fact you need 2 decks for input attenuator (r1 and r2 with value from the lpad calculator) and one deck for each for R6 and R26 (around u5a/b for compensation),... obviously! :