The only time an amp will sound bad is if it is so severely underpowered for the speaker load that you are clipping all-the-time. If you speakers were inefficient, then this could have been the cause. If the replacement amp had substantially more power then that is the answer. Question: Did it sound crappy at low volume as weel as high volume. If low volume was okay, then again you are trying to overdrive the amp beyond its capabilites. Remember that power is logrithmic; to increase 3dB you need to double your power.
However speaking of old amps, I still have a Marantz 1060 which is more than 40 years old ( bought new in 1971) and still plays awesome. The Marantz designation 1060 was a total of 60 watts rms or 30 watts per channel. I am driving Cambrodge Audio S-30 speakers.
Good luck Forest Man!
P.S.- Most bad sound can be traced to the speakers or the HMI