As to the "preamp vs no preamp" question, I get asked this and the answer is you do not need a preamp except for a few things:
To add "color" - preamps ARE colored usually, and there is nothing wrong with color.
To adjust balance and add function, and switch between components. So people go back and forth. It's nice if your CD player gives you the option to "play"
IN answer to your "Tjoeb" question , the Njoe Tjoeb has a very low output impedence, so it will run most any amp, and even long interconnect cables. The output impedence is under 250 ohms. Plenty of voltage at the stock 2.5v out.
The volume control is done in the digital domain, so you should try to keep your normal playing level towards top of the Tjoebs volume control.
If you have an amp that needs very little voltage to drive it, this would not be possible. Except with the Tjoeb. Inside there is a maximum voltage adjustment. 5v, 2.5v (default), 1.25, and .75. This will allow you to get to the point you need.
There is an op-amp kit that will be available at cost for the Njoe Tjoeb 4000. They plug right in, easy. You will in essence have three more flavors for about sixty bucks we guess.