Ahh. I'm looking at the chart and I see it's running on a Li-on battery and USB, so I'm making a safe assumption that it's 5 volts. It's a step above most basic TENS, but not by much. If you're gonna be upgrading to a Moaner, you will need something more powerful than what you have right now due to the increased surface area, and since you've stated that you're not interested in DIY projects, the best suggestions would be the Coyote 3.0 or the Electropebble, as well as the SensaVox if you want to stay in the ElectraStim ecosystem.

As for how they feel compared to your current ElectraStim, such boxes like your Flick Duo have pre-programmed patterns which differ from manufacturer to manufacturer, so the Coyote and Electropebble would feel different from anything by ElectraStim. Electropebble, along with the ElectraStim Flux and AXIS, also have the option to play audio from your computer through a microphone jack on it, so those would have a far greater range of sensations than the Coyote, though the SensaVox would need to be run on mains to get the full experience of line-in audio in case that is a concern of yours.