Converting 2.1V RMS DAC output to instrument level 6.35mm jack
In general, if it's an instrument output it's not hugely well controlled impedance, and it doesn't have to be matched.  I wouldn't even try to impedance match: I'd just use an L pad that presents 1k\$\Omega\$ to the DAC, attenuates down to whatever I've decided to call instrument level, and has whatever (nicely low) output impedance that works out to.
Below is an example 20dB L pad (it looks like an upside-down L -- it got the name before anyone thought to call it a &quot;Gamma pad&quot;, even though it looks like an inverted \$\Gamma\$).  Allmost all you need to do is make sure the resistances add up to 1000\$\Omega\$ and have the attenuation you want.
But before you use this circuit, you should make sure that it'll work with the DC paths to ground as presented -- you may need to add blocking caps for it to work in your particular situation.
For the attenuation \$A\$ as a ratio (and if I'm getting my math right), \$R_1 = (1000\Omega)(1-\frac{1}{A})\$ and \$R_2 = (1000\Omega)\frac{1}{A}\$.  Or just put a 1k\$\Omega\$ pot in there.  Or use a 200\$\Omega\$ pot for R2 and an 806\$\Omega\$ resistor for R1.  Or something.



simulate this circuit &ndash; Schematic created using CircuitLab