That last bit is exactly why I thought to pose the question, hoping to learn something from the experiments of others before me. Your statement that there are multiple variables in play (drive type/brand/etc., interface, cables, power supplies in this case) is the basis for my asking.
My challenge is that I heard a difference between one setup and the other - the internal SSD sounded etched on the top end and more sibilant than the external HDD which smoothed it out and let more detail though. I want to start somewhere in figuring out what variable caused it, and rather than start by spending a pile of money on various parts I might nor might not end up needing to try to even begin putting together a workable experiment, I thought maybe someone here might have already done a similar test to at least point me in a general direction.
In this case, I think I would need someone who has used a single HDD and a single SSD, each one both inside and outside the mini, outside being on the firewire bus and using the same enclosure/power supply/cable setup, to suggest whether the HDD or SSD might be a good place to start in narrowing down my challenge.
Thanks for your thoughts on this so far; your experience suggests so far that the HDD vs. SSD might not be where my change is coming from, so either I could stick with the HDD without concern for it, or I could switch out to the SSD and not lose anything from where I'm at now but maybe pick up an improvement of a lower noise floor removing the spinning hard drive from the listening room (without having to add in a NAS outside the room and all of what that entails).
Thanks again, and to anyone else, please do continue adding ideas if you have them!