When pressed to see if Han is gone for good, Harrison Ford refused to say anything.
the room is not big.. dolby atmos is meant to be set up with 4 in ceiling speakers for best results.. reducing that number will naturally reduce results. however i feel you are right to look at a 9.2 capacity just to give you options and the avrs you mention also are dtsx capable.. so this will (in theory) allow you to use your existing 5.2/7.2/9.2 speaker set.
some yammy refurbs here http://www.excelhifi.com.au/product/av-receivers-en-2/
the rxa2050 (basically up model to the 2079 9.2 atmos/dtsx) is 9.2 atmos/dtsx under $1600
the denons are 7.2 atmos/dtsx around $1500 but i feel they will be a little less as the new denons roll out to the shops. maranz i dont know about
using a separate amp is not my area.. i am sure it will give you a more authoritative sound though.. just not sure if needed in that size room.. others can comment.
I'm no expert, so don't take this as gospel, but I did have a very similar experience to you only yesterday afternoon!
One of my old (second hand) Wharfedale Diamond towers had had a very slight crackling to it, so I swapped the speakers around to see what happened, and the noise followed the speaker. Once I moved them back the crackle was there on and off for a few minutes, and then bam, big drop in mid-ish volume, and on feel, the mid/woofer wasn't working.
Being normal passive speakers (and not super fancy ones), I figure it can only be the driver or the crossover, so I pulled the baffle off and removed the driver. Driver seemed to behave normally when jumped with a battery, so I thought I had better pull out the crossover. Thankfully there was only one inductor on the way to that driver, and it seemed fine, as did all the joints on the crossover.
When I was trying to jam a probe into one of the spade connectors for the driver though, it broke clean through with very little load, so I suppose they were aged, or crappy, or both. Since I couldn't be bothered driving somewhere to buy connectors, I just soldered the wires directly to the wire teminals. It'll be a pain if I need to pull it apart for something, but that's a problem for future me.
Put it all back together and presto, full noise and no crackle!
So, TL;DR: Make sure you have a multimeter, and just pull the sucker apart. Best case it's a dodgy connection like I had, or something on the crossover that's easy to fix. Worst case it's a dead driver (tweeter by the sound of yours) and you can try and hunt up a replacement.
For the sub, I suppose it depends on how you're driving it, but I've always left the crossover at full range on the sub, and controlled the freq. at the receiver end.