Amps are a lot simpler than Pre-pros for sure.
People can nit-pick about this and that. At the end of the day, what I care about are these.
1. Cost. Personally I don't like spending too much. Other people are a lot more willing to spend. ATI amps are usually more expensive than some other amps.
2. Reliability history. ATI has a long history of being one of the most reliable amps.
3. Power output. ATI amps have the reputation of being "Arc Welders". They are considered hardcore amps that can output a lot of power. Both @Irvrobinson
and I have used our ATI 60 Watts per Channel amps to power our Revel Salon2 towers. If buying external amps, instead of AVRs, I'm not buying any 100WPC amps - I'm buying 200WPC amps or close to it. Otherwise, just stick to AVRs.
4. Great Specs - All the ATI amps have great specs. SNR is usually 120dB+, Crosstalk is usually 100dB, etc.
5. Reputation. ATI has the reputation of building amps for many companies as you've heard - Mark Levinson, Lexicon, Cary Audio, Outlaw, Monoprice, and some others that we can't prove since ATI isn't allowed to disclose the brands. ATI also owns ultra-high-end Theta Digital ($20K pre-pros), DataSAT ($20K pre-pros).
McIntosh makes good amps. But there are people who also disagree with some of their technology/philosophy as well. I'll lump this in with the nit-picking. At the end of the day, McIntosh makes good amps and so does ATI, regardless of the nit-picking.
If your wife wants McIntosh amps, I wouldn't disagree with her wishes.