Tell us some more details so we can help guide you to the best system for your needs and budget.
We know you:
* want to download music from iTunes and instantly play it
* want to be able to listen with headphones
So far, I think your best bet is a pair of quality headphones to plug into your computer. If headphone-listening is all you plan to do, this is the way to go. Get an external USB sound card with a built-in headphone amp if the headphone jack on the computer has a lot of noise. If you really want to get fancy, you could get a headphone amp with USB input, or a USB DAC + headphone amp.
* do you also want to be able to play music out loud, through speakers?
* I assume you have a laptop that you use while sitting in the lazy boy, is this correct? Tell us what computer (or iPhone / iPod Touch?) you use to download and listen to music.
* is the computer a Mac, or a PC (Windows/Linux)?
* I also assume you use iTunes to download and play the music, is that right?
I don't know of many USB receivers, and I generally think that is unlikely to be the best solution for your needs. For instance, your laptop might have a digital audio output (MacBook Pros do, for instance -- it is the same jack as the headphone jack) which would allow you to run an optical cable from the lazy boy to the stereo. Then you could focus on getting a stereo that sounds good within your budget rather than finding a system with a quirky USB connection -- you'll spend less and it will sound better.
I've also heard of folks using the Apple AirPort wireless router for their audio systems -- apparently it has an audio output and can be controlled from iTunes on a computer. I don't know the details very well, and may be wrong about what I just said, but it seems like it could be worth looking into for your usage.