RME make excellent converters, I've owned a few myself and was always thrilled by the smooth sound of them, including how well they performed with my DAW system. Total mix is very cool as well.

I'm not familiar with the RME AID-2 Pro Fs but it looks awesome, I suspect it would be noticeably better sounding to the PreSonus.

The RME ADI-2 Pro FS is indeed a high-quality box, and performs excellently within the bounds of what it is designed to do. Unfortunately, it is essentially a 4 input channel unit (2 analogue in and 2 via RCA/Optical) and does not so easily work in conjunction with other multi-channel boxes.

If you are looking for an expandable high-quality USB interface, I recommend that you look at the Audient ID44 MkII. I run that unit along with a pair of ASP880s (like the one you have) to give me 20 channels of input at 48KHz, dropping (for ADAT capacity reasons) to 12 channels at 96KHz using a single ASP880 plugged into the ID44. Having balanced inserts on every channel on the ASP880s is a real luxury, and allows me to feed my API 3124+ and other external pre-amps directly into the ASP880's ADCs without having to route through a second pre-amp.

As a mainly Windows user rather than Macintosh, I have never tried the Presonus Thunderbolt Quantum interface, so I've not been in a position to compare the audio quality of the ASP880 side-by-side with the Quantum. However, of all the pre-amp/converters I have used, I have yet to find any that can match the audio quality from the newer (MkII) Audient boxes in terms of what you get for your money. I'd be very interested to know if you notice any difference between channels converted on the Quantum versus those on the ASP880.