Is it worth replacing an old amp?

Amplifiers sound different. Replacing your old DSP-A1 with a new amp o an old amp could change the sound and improve or worsen the audio quality. That being said, there have been significant advances in amplifier technology over the past few decades. Whether or not those advances sound better to you or not is something only you can decide.

@divby0 some speakers are easier to drive than others. In that situation, an amplifier that sounds good with an easy-to-drive speaker may have problems with a difficult-to-drive speaker and therefore will sound bad.

Likewise, there is a degree of matching that will take place. Let's say you have a speaker that has harsh treble. Using an amplifier that rolls off the high frequencies could make that speaker sound better. But that same amplifier with a speaker with smooth extended treble will make it sound like you're missing out on some of the music.

As others have said, you need to go listen to some amplifiers to decide if it will improve on the sound of your existing system. Just as what you hear when replacing your amplifier will depend on the rest of the equipment you are using, plus your room and listening situation, later on if you decide to replace something else like your speakers, the result will again depend on everything else including the new amplifier you got.