You're already used to having a lot of power, so I second Sufentanil's call for big brawny amps like the Krell 600 watters or the Plinius SA 250 - the small and expensive Pass Class xa series amps are beautiful in smaller applications but just aren't going to give you the volume you're used to. Don't know what ARC gear you are looking at, but Modwright doesn't make anything I'd use in a room that size.
Other Plinius possibilities that pop up for sale here with some frequency are the SB300 or SB301. I use the latter to drive Revel Salon 2s in a nearly 6,000 c.f. room, and it has a nice decay with good detail without sounding clinical or cold. The earlier model SB300 is essentially the same as the SB301 with some cosmetic differences and there's currently one listed for under $4K, and also an SB301 for $5K.
With your budgeted amount and willingness to buy used you're actually very well-positioned to make a serious upgrade. Just a few of the other possibilities available here in that price range are a Mark Levinson 532H for just over $6K (I auditioned my Salon 2s with that amp and really preferred the Plinius, but they are detailed), a Pass x350.5 that would fill that big room of yours nicely, a pair of the aforementioned Krell FPB600 monoblocks for just under $6K, a Krell Evolution 302 for just under $6k, etc. etc. etc. Happy hunting!
