Preamp or not.

If you are getting the volume you need from your existing setup, then you don't need a preamp with any gain.  

The problem with controlling the volume from the DAC is that this is often done by reducing the number of bits going to the DAC, which reduces resolution.   An analog volume control doesn't do this.   Since you are using an external DAC with your Node 2i, the only way it can control the volume is to reduce it digitally before the DAC processes the signal.

I can recommend the Schiit Saga+ preamp. It is zero gain, and offers both a passive mode and tube buffer option. The volume is controlled by a stepped resistor network so the balance between channels is very precise. It also comes with a remote.  Cost is only $400 and they have a two week trial policy but do charge a 5% restocking fee if your return it.  If you need extra gain or want something fancier, the Schiit Freya+ preamp is $900. Both units are popular with lots of good reviews.

Power and sensitivity should not be an issue for you. The Pass amp needs roughly 1.25 volts input for its full 25 watt output at 8 ohms. Your Topping, at 2 volts max out, has more than enough to drive the Pass, so with your efficient speakers, you’re going to need to be cutting volume quite a bit (unless you are a full tilt all-the-time head-banger.)

So, the question becomes, what is the best tool for controlling the input voltage to your amp? You don’t need any more gain, so that points to a totally passive or unity gain analog preamp. This would allow you to run the digital signal at 100% (no bit loss) but still control the volume to your liking.

Besides the Saga+ mentioned before, Schiit also has a totally passive volume control -- the Sys, for only $49. But, it has no remote. Others have suggested other excellent preamps, though you’d want to consider the gain issue in choosing one of them.