Since you mentioned that you were interested in internet radio and the unit in question was an AVR I gather you are not looking for major video over the wireless connection you hope for, rather, you want a reliable connection primarily for internet radio and just general connectivity for the AVR (firmware updates, etc.). If this is true, wireless should still be fine for those purposes. I would look at your web configuration menu for the spare D-Link router - you want to look for the ability to switch it into a client bridge mode. This would allow it to act as a client adapter which is what you want; you would attach the spare router to your AVR via Ethernet cable and use the spare router in client bridge mode to connect to your main 3700 router.
If you cannot find a mode option in the spare router's web configuration then it may not support that mode with the standard manufacturer's installed firmware. You might still be able to use the router, however, if you are willing to try a different firmware option such as DD-WRT or Tomato. Google both of these and check through there lists of supported hardware models and see if your spare router is supported. If your spare router is just sitting collecting dust otherwise, this might be a good option. Study the how-to's on the DD-WRT and Tomato websites and see what you think; there is the risk that the firmware update/change could brick your router but if you are careful and follow the directions for proper rebooting and resetting I don't think you'll have a problem. Worth considering anyway, especially if your spare router is not being used for anything else.
Also, for what it's worth, I stream alot of video via wireless and have not had a problem; of course, in my case, my internet connection is modest at 5Mbps down and internet streaming isn't necessarily taxing unless true high definition streaming video is constantly needed. Plus, most of my streaming is done with Roku boxes streaming Netflix and occasionally Amazon VOD so that is also not significantly taxing though what supposed "HD" content I do stream with these devices seems to work fine. My default wireless band for this is 5GHz with fall back to 2.4GHz if necessary. I'm using a Linksys RV042 router and a Netgear WNDAP350 AP - been running stable with no reboots needed for several months now. My network is modest, admittedly, consisting of three PC's usually, occasionally more, plus two Roku boxes and five printers (four connected to an old WGR54GL configured in wireless client bridge mode using DD-WRT firmware and one with its own wireless connection) but this has been my most stable setup by far!
-Mike