I had the same question on overloading. I'm running the PS-Audio PerfectWave DAC MkII (still shopping for a replacement). That DAC's output is quoted as 10 volts, unusually high for a DAC. I've had no problem using the Tortuga LDR1B (or the LDRxB which I also auditioned) with that input level. I'm running my system balanced throughout.
I've also tried the Tortuga in other systems with DACs rated in the 2V output range. The only difference was that those systems required a bit less attenuation.
I've found the selectable input impedance to be very useful to get the most out of a system. For mine I using input impedance settings of 7, 8, 10, 15, and 20 kohms. Given that the impedance that the source sees using the balanced input is doubled, that's actually 14, 16, 20, 30, and 40 kohms.
Using the PS-Audio DAC as source, the 20 kohms setting is a good middle ground. At the lower impedance settings, the bass becomes a bit more full and articulated with the overall sound becoming more lush. Below 14 kohms more harmonics are present though I suspect this may be a bit higher harmonic distortion in the DAC when loaded into low impedance. At the moment I'm using 14 kohms input impedance (shown as 7 on the display). At the 30 kohm and higher settings, the bass is leaner. At 40k the soundstage starts to have a bit less depth.
I expect that an optimal impedance setting will vary depending on the system in which the Tortuga is installed. It's a great feature that this can be interactively customized and controlled by the remote.