I’m going to try and withhold my MQA bias here and give you a technical answer based on my understanding of MQA…
A MQA file uses some of the 16 bits of redbook to hold info about the file. It actually reduces the “CD” quality of a file down to 14 or 13 bits (somewhere in there I forget the exact number).
The first unfold (which can be done in software and is what Roon is doing) takes those bits and reconstructs, at best, a 24/96 kHz file. You get the “MQA mastering” as well as the reconstructed 24/96 higher resolution file. It should be noted that anything beyond the ~22kHz frequencies is reconstructed from a “lossy” combination of compression and CODEC magic. The high resolution portion of the file does use compression to reduce file size.
Adding a MQA DAC includes the proprietary MQA filters in the DAC as well as an additional reconstruction (unfolding) of up to 24/192. There is a hardware requirement here because there is a decode operation which cannot be accomplished in software in real time.
It’s this additional decode to the 192 resolution plus the MQA filters in a “MQA DAC” that will resolve the entire file. Do you need a MQA DAC? It really depends on if you are onboard with MQA and like the way the MQA filters sound. A comparison in the method you suggest may be problematic because no two DACs will sound the same. Your question is one of buying a MQA compatible DAC or not. A MQA DAC means it’s using MQA filters so it’s going to sound different than a non-MQA DAC and that is regardless of the files resolution. Basically, I’d find it very difficult to compare a second unfold to the first unfold as there are too many variables involved during the second unfold which will color the sound. Either you’ll like the way the sound gets colored or you will not. That has everything to do with the DAC + the DACs reconstruction of the compressed high resolution + the MQA filters at the second unfold.
But, the short answer, if you’re all in on MQA you’ll want a MQA compatible DAC. If you’re already getting excellent sound and are not ready to be all in on MQA then I suggest you keep what sounds good. Technically, of the two, a 24/xx file vs. a “MQA’d” 24/xx file that’s been folded and now needs to be unfolded and with both files coming from the same master… the 24/xx non-MQA should sound better as the high resolution (those beyond ~22kHz) has not been compressed. However, and again, applying the MQA filters may be preferred and therefore can sound better to some listeners.
There is also the question of how you get your music? If you primarily stream Tidal and want the highest resolution possible then a MQA DAC is required. Tidal is all-in on MQA and that is their high resolution format.
Personally (bias re-engaged): I am currently shopping for a new DAC and have decided that MQA is not a requirement for me. I prefer, purely on a technical level, DSD or PCM without the MQA process. However, a handful of DACs I am considering include MQA anyway but it isn’t influencing my purchase decision.
Good luck and happy listening.