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:
http://www.parts-express.com/dayton-...ifier--300-784
http://www.rjbaudio.com/Cerberus/cerberus.html

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?