I have owned both Z7 & 3900 (the Z7 longer than the 3900) .... and can whole heartedly say Z7 all the way.
BUT … Either way be forewarned - INSURE those units, and pray if that HDMI board goes it does so in a lightning storm. It will cost close to that price to replace the board alone IF at all available (I honestly think they might not be, so it may even be scrapped) as it is one of the very first versions of hdmi and the production run was very short (as with all first generation boards around 8-9 years ago)
Effectively it would be worth the price, IF it wasn't for the risk ito the HDMI board. I sold mine for R24000 around 8 years ago, when the retail was R40K.
There is no replacement to either, but closest replacement (for either ito insurance purposes) would be the RX-A3060. IMHO the Z7 was better though (how can it not be considering that almost 10 years later the said RRP on the 3060 is only 20% more and at the time the Z7 cost almost 50% more than the 3900??? ) so effectively I would aim to the pre-power (No insurance would agree to that though, purely due to the fact that the z7 had only 7 channels).
As far as I can remember the z11 (at twice the price of Z7
) was basically the just an upgrade in channels, which translated in zones and all round versatility. For that the replacement would absolutely be the Pre-power.