Well, currently I'm playing things as in my signature. I love the convenience of Apple Lossless via the digital out of an Airport Express, all controlled with an iPhone.
However, last night, I connected to my wife's Library on her MBP and no sound was coming out of my speakers. I noticed on the iPhone that the volume was turned all of the way down. I always leave my machine's iTunes' volume at 100%. This was an unpleasant discovery.
Anyhow, the simple conclusion is this is not a lossless setup. It can't be if I have volume control via iPhone/iTunes.
Now what do I do???
Sounds like you want hard proof(especially given your username!

). Given it's consumer market, I doubt if you will find the exact proof of the lossless accuracy, but this
Benchmark Media wiki entry might be enough to hold you over. See the section titled "Article from Feedback Newsletter comparing iTunes 6 vs. iTunes 7" It speaks of 24bit dithering on the volume. Which is on the itunes side not the AE side.
I see in your signature you wonder if your Denon is the weak link...
Is it perfect? Doubt it. but with the AE it is a simple/low cost set up. I used it for a while: Denon AVR 4306 with an AE via Toslnk. I think the AE is the weak link if you are really looking for optimal sound. I had problems with drop outs. I still find the AE fine for background music in other rooms.
I've also tested my 4306 (using it's internal DAC) using optical Toslink directly from my mac (itunes) and preferred my USB DAC with high rez non hi-rez files.
So you might want to look into removing the AE from your source chain.
Look at
SEARCH here using "airport express" as search words to see previous discussions of its use.
have fun, ed