There are a lot of different, and valid, choices. It's really going to depend on how good the components are and how well they mate with the gear you already own. For me, the most important part of a system to get right is the preamp. Most people don't agree with me in this. The only thing I can say, is that its one of those things you need to hear for yourself. The difference a really good analog preamp (tube or SS) makes in your system is huge. There's nothing subtle about it.
When you are talking about a digital preamp, I take it you mean a DAC with a volume control and possibly more than 1 input. You can get some very good sound that way also. Keep in mind that ultimately, the digital signal has to be converted to analog at some point, so even a digital DAC/preamp will have to have an analog section before it gets to your amp.
In the end, my advice would to be to pick a component based on how much you like it first and the actual design 2nd. Looking at your system, my best guess would be that your weakest component is the Jolida preamp. If it were my choice, I would replace that first. Your Sim is an excellent component and I think you will get a lot more out of it if you run it through a better preamp. Like I said, though, there are a lot of great options.
