I can't say that I have ever heard the RX-Z1 but I do know that AV receivers of that era relied a lot more heavily on their own "custom tweaks" and even when decoding what was somewhat scarce -- discrete surround content delivered via the complex wiring of analog S-Video and separate digital audio signals, it seems the design of pre-THX AV receivers left lots of rooms for unique interpretations of things like intra-channel time delay...
That said the amplifier of the top of the line Yamaha and its DAC were supposedly superb --
http://www.hometheaterhifi.com/volume_10_4/yamaha-rx-z1-receiver-11-2003-part-2.htmlIf you pair the 975 with an appropriately muscular amp and don't need any of the legacy connections you will probably be amazed with the accuracy of its surround decoding, and frankly even teamed with less than a beast of an amp you should never really "outgrow" the power side of the separates path, quality units last essentially forever and can generally team with any new technology. Will you need 4K someday? How about HDMI 2.0? Of course the 975 is not really designed with the same "master control" mindset of your receiver and if you want /need a "hub" to handle complex switching / universal remote there are other ways to achieve that. (The Outlaw store sells Marantz 7000 & 8000 series products for that reason...)
I am not sure the Nuforce product is really all that much better, despite a price tag that is about double the 975. It seems the implementation of its "auto eq" is not very effective --
http://www.stereophile.com/content/music-round-63-page-2 To me that explains why Outlaw decided against an auto setup AND why firms with proven expertise like Audyssey are the "go to" source of the proper implementation of tricky room correction for smart manufacturers like Marantz/Denon, Onkyo/Integra, NAD, Cambridge Audio, Wisdom, McIntosh etc...
Not really sure where the Emotiva product falls. While it is equipped with more a few more legacy connections and a wider range of settings than the 975, it is not really "top of the range" the way your Yamaha once was... It is fairly close comparison at list price, but when you look at combo packages the "Outlaw pricing" sure is hard to beat.
The thing too if you really want to judge a product like this on "detailed, quality, clean" sound the implementation of that can be achieved with a fairly simple approach. Many folks would say the "minimalist mindset" is really what the 975 epitomizes, taking the "simpler is better" approach has been a rare way to tackle the home theater.