Is it worth replacing an old amp?

Divvy,

I too had a 205. When I went to Streaming i sold it and bought a DAC and a Streamer. Now talk about an improvement in SQ! Wow, even the wife commented how much better it sounded. And that was using a $750 Ares 2 DAC. 

Not sure how much you want to spend.

I have no experience with 5.1 receivers, sorry.

I feel the front end of your receiver is still worthy of serving you well. You would benefit driving your speakers with amplifiers that can effortlessly handle their low impedance. I followed a suggestion to select an amplifier well above the recommended wattage so a strong passage or sound effect would play without reaching the clipping level of the amplifier. No need to play at high levels to hear the benefit of an amplifier upgrade.

My system is a Yamaha RX-V777bt running as a preamp, Parasound A21 driving AR-303 L/R, ARC D200 in bridged mode driving an AR2c Center, ARC D200 driving a pair of Avid 103 for surround L/R, Kenwood Basic M1 driving a pair of ENERGY Connoisseur C-1 as Back Surround speakers. I upgraded the Bluetooth receiver to an XQ-50 Pro Bluetooth 5.0 w/ LDAC, digital out (coax) to the RX-V777. Audio from the LG 4K TV via TosLink.

Something that people haven't mentioned is that over time, the capacitors in the amplifier section go bad and need to be replaced.  That alone will likely improve your SQ. 

My amps are 36 years old. You couldn’t pay me enough to give them up.