After voting in the poll, I went for Amazon Music's "HD Ultra" tier. My Topping E30 DAC is now set to be capable of playing back at a sampling rate of 384khz. Looking at the info at Amazon Music indicates playback with as high a resolution as 24/192. Two things worth noting. First, I don't hear any difference in the quality of sound that couldn't be explained by different mastering or mixing. Second, Amazon Music HD is a RAM hog, my cheap & cheerful Acer Aspire 5 has only 4GB of RAM, so dropouts are frequent. I'm guessing my laptop can't multitask Microsoft Explorer and Amazon Music without major irritation. I'm going to experiment, run Amazon Music HD without surfing the web at the same time. Fortunately I-Tunes works smoothly and I have plenty of Apple Lossless files. Also, it looks like it's pretty easy to upgrade the RAM with this laptop, so I might do that. Amazon Music has a deep catalog including a lot of obscure Classical music, so it's worth $10 a month to me.
Bottom line: while Hi-Rez files sound good, they don't sound better to these ears than standard Redbook, which is higher rez than LPs. Note that I also have around 60 discs of various Hi-Rez formats, SACDs, DVD-A, Blu-Ray. While some sound very different, it's clearly on account of remastering/remixing. The Grateful Dead "Workingman's Dead" DVD-A uses different 'takes' for its remix than the standard CD. There's some obvious performance differences.