Variable gain preamp ?

It would depend on the amp and the efficiency of your speakers, as well as the ouput voltage of your source (most digital sources are 2V but some are not). The more sensitive your amp is the more gain it has. The more efficient your speakers are the more easily it is to drive them. Too much gain in your system and you may not have much range on the volume control. You see quite a few threads where people mention they can't get much past 9 o'clock on their volume control. Operating the volume control between 11and 2 o'clock seems to be the norm from what I've read. Some of that has to do with the specific type of volume control. There is a theory that some volume controls have more linearity towards the top end of their range. I will assume your source is 2V output and I see your speakers are not so efficient, so my guess is anything 1V and under for amp sensitivity and you would benefit from the variable volume control. In fact if price is not an issue I would get it anyway just for the flexibility.

I am only running digital . a Mac mini to a Tranquility Dac .

A Klyne 7 for example has that feature. The advantages here are
- you can adjust the Input from a connected unit to your preferred level
- you can match the Preamp to the Amp you want to use
Overall - when done right - it is a super feature.

I have a slightly different example. I had an older Gamut preamp with 6dB of gain. My Gamut amplifier has selectable sensitivity to mate with different preamps. To match the it's brand companion I used the most sensitive position. I upgraded to Parasound JC-2 which has 14dB of gain. I adjusted the sensitivity lower on the amp to match the preamp. Now, the JC-2 has adjustable left & right gain, but manual suggests these are used more like a balance control, but I think you could use them to dial back the gain also.