If you ever get a chance to try a NOS DAC with/without software upsampling (or if you have already), I'd be curious to see if your impressions match what some others have noticed or not.

I see. Well, many claim software upsampling "fixes" aspects like you noted. I've noticed some level of difference but wanted to get opinions from people here to verify.

I'm not so much talking about critical reviewing or comparing gear on strict terms, but I think software upsampling (of which you can, in theory, choose nearly any method you'd like) coupled with a NOS DAC is a viable way to listen critically and for pleasure. It's taking a commonly used DAC function and moving it to a different part of the chain, though you have the option to opt out entirely if you so choose. In my mind, I'm hesitant to liken it to something like EQ because of this, but I can understand if you disagree. I personally like having such an expanse of options to dial in whatever works best for my ears.

And, as mentioned, I have read other subjective reports, of which I can't vouch for their credibility, containing similar impressions to yours that were later mitigated with the use of software upsampling.  I distinctly remember someone saying the Octave sounded "mushy" at 16/44.1 and without software upsampling. The potential variability of software and upsampling methods used does raise further doubts and questions (not to mention they could have bad ears or just be idiots), but the relative consistency of the reports is interesting enough that I think it warrants further research. And it's not to hard to believe, really, considering a higher sampling rate does lessen the treble drop-off in the audible range. That alone has a noticeable impact on the final sound.

Most of you here seem to be a bit more critical and honest in your evaluations, hence the reason I was particularly interested in gathering thoughts from anyone that might have also experimented with software sampling and NOS DACs. I can hear a difference with/without software upsampling, but I still have a while to go before I'd feel comfortable making definitive statements one way or the other, so I really value others' impressions as well.

And if some of my test tracks are 24/96, 24/352 and DSD which sound the same as 16/44 on the Metrum to me, where does software upsampling fit into that theory?

Well, if that is indeed the case, I'd say that is evidence against the claims others have made, and that's the sort of information I was curious about gathering and think is valuable to all listeners/potential buyers. I think software upsampling can bring some benefits to red book music on a NOS DAC, but what I've heard is close enough it could just be placebo (perhaps minus the slight treble lift). Certainly not the dramatic change some make it out to be that will make all the difference in the world to those that don't use it.

I preferred the Metrum Quad with JRMC up sampling. Seemed less hazy, more clear in the bass, and slightly more detailed. I wonder if this is because the ultrasonic shit is pushed up further where the analog filter would be more effective?

Similar in some respects to DSD to PCM conversions where I prefer the most aggressive ultrasonic filters. The DSD to PCM conversions with the DSD hash intact from 24k upwards sounds similarly hazy to NOS and no software up sampling on the Quad.