This is about WIIM mini vs Pro but more perhaps a naive newbie question.
Can the WIIM pro stream any music or sound from my Apple desktop computer (2008 Mac mini) through the ethernet connection, and if so, would sound (music) be any better quality than streaming the same music from the same source using bluetooth/Airplay2?
First, Bluetooth and Airplay are quite different. Bluetooth communicates between two devices using a special low power radio. It always transmits compressed music. Airplay2 transmits over the local area network (LAN: either WiFi or ethernet) and can transmit uncompressed CD quality audio.
So there are two factors here: compression vs. uncompressed, and the means of communication. Theoretically uncompressed CD audio is better than compressed Bluetooth audio. However, most people can't tell the difference in a blind A/B/X test. But if you can, or think you can, then you will want the uncompressed audio through Airplay2.
Second is the means of communication. Bluetooth uses low power radios with limited range. Wifi uses much more powerful radios so can communicate over greater range. However, depending on your equipment, the wifi may be weak or subject to interference. Ethernet carries the same network data as wifi but over a wired connection, and generally has the best data rate and data integrity of the three options. Airplay2 communicates over the LAN (either wifi or ethernet). So theoretically, the best and most reliable choice (of the options discussed here) would be uncompressed audio over Airplay2 over ethernet. However, if you have a good wifi, there is no practical difference between wifi and ethernet.
Both the Wiim Mini and Pro can do what you want, except the Mini has no ethernet. If you have decent Wifi then that doesn't matter.
I used to use ethernet to stream audio and video to some rooms upstairs where the wifi was weak, but then I got a better wifi system and now I don't bother with the ethernet.
I generally stream music from my apple computer to my old Denon AVR receiver (AVR-687) using a relatively cheap Taotronics bluetooth receiver. it works fine, but drops bluetooth constantly and have to re-pair devices every couple of days if I don't use things for a few days.
So you could use the WiiM and stream to it with Airplay instead of bluetooth, using either Wifi or ethernet.
So in general, if I got either of the WIIMs, would I get any better connectivity, would I get any better sound quality, or since the WIIMs would be hooked up to the old AVR-687, would that receiver limit any sound boost that the WIIMs might provide?
Yes, yes, no. Well made audio amplifiers have been "transparent" for a long time. Unless you have some specific issue with it, your AVR is fne.
You would have the choice of connecting from the WiiM to the AVR by analog or digital (optical or coax). If you use any of the DSP functions on the AVR, such as equalization, tone controls, sound field effects, then it makes sense to connect via the digital connection. The Mini has only optical, the Pro has optical and coax. Both work the same.
A lot of folks here will insist that you need a better DAC than the one in the WiiMs. I doubt most (any?) of them could tell the difference in a blind test. I can't, so I use the analog output from a WiiM Mini in my office system, right into a Aiyima A07 amp. Sounds great.
And finally, I have been looking to perhaps get rid of the Denon AVR and just move to an integrated amp with some type of network or streaming capabilities--would like to stay under $1,000 if I go this route--have been looking at the Denon PMA900HNE and possibly the Cambridge CXA61 or 81. I am not wed to Denon and was first interested in maybe Marantz, but need A/B speaker capabilities which the Marantz's seem to lack.
I think it's cheaper (and better) to use a dedicated streamer like the WiiM. That's the part that will get obsolete fastest - if it does you can just replace it. And WiiM seems much better at software support - something most conventional audio equipment companies are very weak at.
Another thing I prefer about streaming over the network instead of Bluetooth is that ringtones, system sounds, notifications, etc. don't get sent to the audio system when I'm listening to music. The default with Bluetooth is that all the sounds from the computer or phone get sent to Bluetooth, which is annoying.