I'm using an LDRxB just back from the v25 and OLED upgrade. Low amplifier input impedance is of course relevant, but my experience is that the input impedance setting of the Tortugas is very important to the voicing of your source equipment. I've been using the Lampizator Golden Gate DAC and recently upgraded to their Pacific DAC. These are both vacuum tube designs, using a tube rectified power supply and a quad of tubes for the output (one per each phase of each channel) using balanced output.
Both of these DACs benefit from seeing a high input impedance controller/preamp. These are the impedance settings on my Tortuga: 20, 65, 75, 78, 81, 84, 88, 91, 93, 95. I stay in the 80's and 90's range. The steps may seem small, but each has subtle effects on the DAC's sound.
At higher values bass is tighter, soundstage more focused. Treble begins to be a bit subdued at the highest settings which can be beneficial with bright or hard sounding recordings.
Lower values in can have more bloom and can open up the soundstage when called for. At the low end of the range (in the 70's for my system), the frequency balance can begin to tilt upward. Sometimes this is just what a particular recording likes.
This optimization of source equipment performance is just one of the great features of the Tortugas. It lets me optimize the sonic signature of my system, all from the listening position with the remote. I have yet to find any other controller/preamp that provides that degree of control over my system.
By the way, the improvements with the v25 upgrade are well worth it. I was able to compare my Tortuga before ordering the upgrade with a friend's Tortuga that had the upgrade. The v25 was more dynamic and transparent. The OLED upgrade adds a lot of functionality and ease of use.