My comments come from experience of owning a Pioneer A400 as well as many, many other amps.
The A400 remains competitive with any modern budget amplifier, and outclasses many of them. I certainly wouldn't change it for an entry-level Marantz as (having also owned a PM6007) as that would, IMO and despite the Marantz being capable, be a noticeable step backwards. As others have said, the only potential issue is age, but I'm regularly using an amp that's 47 years old with no issues whatsoever.
The most affordable amp I would suggest trying is the Rega Brio, which you are likely to find fuller and more fluid sounding, though your current speakers will only show some rather than all of the difference. Another good option would be the Audiolab 6000A or the LEAK Stereo 130 (both of which I have heard). Perhaps the one I would choose to replace an A400 is the Cyrus ONE HD - a cracking amp that shares qualities with the Pioneer and takes them up a level.
A bigger budget will open up more possibilities, but these are solid starting points.
Hope this helps.