Is it worth replacing an old amp?

Hello,
If your amp is working you will notice a much bigger difference in the preamp. Emotiva has a two channel preamp with HT bypass. Used it’s about $800. This is the start or bottom of preamps but has awesome sound and capabilities. On the high end but doing the same is Ayre or Hegel. I think MACs have this feature too. Try to borrow a two channel preamp so you can hear what I am talking about. I do this right now with the Emotiva ups1 preamp with a Marsh Sound Design. I would pass on Rogue Audio with bypass. When I tried their HT bypass it had a 10db loss in volume due to you power it off to engage the bypass. This store in the Chicagoland area lets you try before you buy. https://holmaudio.com/
They do not carry Emotiva so you will have to trust me it’s awesome with tons of different amps. I hope this helped. 

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.