Get the denon x1800 while it’s still available and you’ll have to worry about this question in 2035… come to think about it, maybe get two.
There’s always Sony too, who still put out pretty good receivers.
eBay. Plenty of excellent vintage un-shittified sound equipment for sale. You may need to re-cap the amplifiers, but the speakers should still sound perfect.
You're doing 5.1.2 on vintage equipment?
Buy enough speakers and you can do x.y.z
Can you? Doesn't the receiver have to know how to decode it?
It's absolutely possible to do whatever amount of channels you want, just buy enough amplifiers. You may need other equipment to "decode", but it's easily possible to get a media PC with 7.1 analog audio outputs, or more if you really wanted to do it for whatever purpose you need. None of this is really impossible, it just takes money.
> but the speakers should still sound perfect
speakers are wear items
I have a set of 1970s Sansui speakers that are still going strong. Nothing wrong with them. I use them every day, all day long, and have for decades.
Minidsp flex ht or htx paired with a buckeye 6 channel amp. As cheap as you can get premium sound quality. Not cheap but you get the software control you actually want via the minidsp