I don't know jack about Alpine decks these days, so I wouldn't have a clue as to its level of compatibility with other portable devices. Did you control your old iRiver device from your h/u? Or did you control songs with the controls on the iRiver itself?
This is important because quite a few DAP and PMP devices these days go into a passive mode when connected to a recharging source like a USB port, requiring all data control be done on the h/u. This assumes that the deck can actually UNDERSTAND the attached device. For instance, the JVC h/u in my car has no problems with one of my iPods or a flash drive, but attach a Sony Walkman PMP or a Creative Zen or Zen X-Fi to it through USB and it's helplessly lost. A lot of it has to do with the deck's ability--or inability, as my case may be--in interpreting the operating system and/or storage structure of the portable device. A lot of older devices seemed to be easier to interface, whereas perhaps the added features and complexity to current-day portables seems to have mucked up the data exchange thoroughly.
Even though with both my Walkmans and Zen X-Fi I have drag-n-drop file capability, my deck won't seem them as a connected drive like it would a flash drive. Instead my JVC just seems them as some sort of alien device that it can't understand. Without knowing more about your Alpine, it's hard to give a definitive recommendation on what brands and models of DAPs or PMPs for you to look and listen to.
That said, on the issue of SQ alone, it's hard to go wrong with any Sony S-series player. They're superb sounding, and have a fairly easy-to-use interface. Since they're flash-based, have a relatively petite footprint, and they're lightweight which make them easy to wear. They range in capacity, but I believe the S-series tops out at 8GB. If you want more storage, you'd have to wait for the upcoming X-series (maxing out at 32GB) or consider a NOS model of the now discontinued A-series (though that tops out at 16GB). Jasmine France of CNET seems to think a LOT about the X-series based on a preview model she got to try. It's supposed to be out within the month.
I own the 16GB A829 (for about 10 months) and aside from a nit with the inability to make on-the-fly playlists I couldn't be happier with it. Sony now has that model in limited quantities in some of its Sony Style B&M stores for around $160 these days, about half of what it used to sell for when first introduced. It's great for working out, because it's compatible with stereo Bluetooth headphones (no wires, but perhaps at the cost of questionable SQ). But again, I'd be leery about whether it's going to work with your Alpine.