I don't think the 990 has much value, maybe $200 at most. The last one to sell on eBay went for $137. The 7700 however (a rebadged ATI 2007) has kept much of its value and one just sold on eBay for $1285. Other rebadged ATI amps with fancier names may have sold for more. An ATI 2004 is currently for sale on eBay for $1500. I would try to sell them separately given the outdated technology of the 990.
I have to say if I was going to set up a HT at a new place my ATI 2005 might be the one piece of AV gear I'd take with me. Amps last a long time and quality ones like the 7700/ATI 2007 are expensive to replace. You will spend much less moving your 7700 than the additional cost to replace it.
Outlaw 976 | ATI 2005 | M&K 850s Left, Center & Right, Surround-55s, MX-70B Sub | Harmony 950 remote | Panamax 5100 | OPPO UDP-203 | LG OLED55B7