The S9 Pro (not S9) running in balanced mode has plenty of power for the RT. I've been using it side-by-side with Topping A50s desktop amp and E1DA 9038s G3 dongle for several months. with many A/B comparisons. The S9 Pro sounds great, and is hard to distinguish from the powerful Topping desktop amp even with demanding material, such as complex orchestral music or electronica with lots of sub-bass. The E1DA 9038s in balanced mode also has enough power, but the amp section seems to generate a slightly darker, less-open tonality. (You might not notice it except in A/B comparisons, and even if you did, you might be able to EQ it.) I'm familiar with what the RT's sound like with either inadequate or simply not-quite-ideal power sources. However, either of these two dongles will work.
I don't know if balanced mode is necessary, but it may help. That said, I pointed staticV3 to some independent measurements of the S9 Pro that suggest to him, tentatively, that at the lowest impedances (like the RT) balanced mode may not help. If true, the S9 Pro single-ended would work just as well. Check with him if more measurements (e.g. on ASR) appear. Running balanced mode does mean a balanced headphone output and a balanced cable with DCA-style Hirose connectors from someone like Hart Audio Cable (much cheaper than DCA; I like the ones I've gotten there). The S9 Pro has both balanced and single-ended jacks; the E1DA 9038s runs only balanced, but there is the single-ended 9038d as an alternative.
If I can get a chance, I'll try comparing the S9 Pro A/B in single-ended mode to the Topping in single-ended mode and the E1DA in balanced mode (since it only runs in balanced mode).
[Sorry, I just noticed that this thread is headlined only for the S9, not the S9 Pro. If this post is a distraction, sorry!]