Budget Upgrade for 2.1 bookshelf system

My old receiver just died and I was thinking of upgrading to an integrated for around $500.

Speakers are Usher S520
Usher S-520
Description: Two-way front-ported bookshelf loudspeaker.
Drive-units: 1" UA25-10 silk-dome tweeter, 5" KSW2-5029B polypropylene-cone mid-woofer. Sensitivity: 86dB/W/m.
Nominal impedance: 8 ohms.
Frequency response: 55Hz–20kHz, –3dB.
Power handling: 50W.
Crossover frequency: 2kHz.

I also have this DIY subwoofer which has line and speaker level inputs:

This is in a large living room, but unfortunately speaker upgrade is not in the cards.

Source will be .mp3 and FLAC files from a regular computer- I had been using Chromecast Audio which has a TOSLINK output. We also stream Pandora and things like that from our phones.

I had a HK 3490, and the new HK 3770 looks like it would fit the bill but perhaps I could make a step up? Also considering NAD D3020. Anything else I should take a look at? Would it be worth it to look at used in this price range?
your speakers are slightly harder to drive, so you'd be well served with an amp that can handle more challenging loads. spearit is listing a nad d7020 for $599--it has an inboard dac for your computer and more juice than the d3020. no affiliation.

Thanks for the link to the Peachtree Decco65. I didn't really need one, but at the 45% off price, I jumped on it.
I was also concerned about he power capability of the D3020. The Decco65 seems like a good buy. How is the reliability on that unit?
I ordered the PeachTree Audio Decco65 from Upscale Audio in California. It shipped and arrived in Cincinnati 4 days later. That is excellent service! I've got it burning-in in my big rig right now. Out of the box it sounded pretty flat, but with about 10 hours on it, it has improved immensely. I'm just streaming Radio Paradise to it right now. I'll do some more critical listening in a couple of days. 

At the current closeout price, I think it's steal considering it's multi-input DAC capabilities.