Tech normally moves too fast to bother with the cost of recapping an old AV receiver.
Speaker though is a different situation. This is to do with how ferrofluid ages.
Ferrofluid became popular in the 1980s. Lots of brands jumped on the bandwagon and started to fill the magnet gaps of their drivers with ferrofluid as a way to help deal with voice coil heating. What perhaps wasn't considered though is ferrofluid dries out over time. Any older speakers that have either been in storage or simply left unused for a year or more could well start to display high frequency problems due to the fluid becoming a bit sludgy. What happens is it impedes the movement of the tweeter coil at higher frequencies. It's sometimes mistaken for blown tweeters, but if you know the crossover point of the speakers and have an AV receiver with graphical displays of each speaker response then you'll see that the HF gradually fades away rather than dying after the crossover point.
This is something I've only just encountered, but it does explain why some once great speakers seem to have lost their shine.
In my case I was doing some work with a system using KEF TDM THX Reference home cinema speakers. These came out in the late 90s. The front soundstage in this system sounded dull, and at first I thought all three tweeters had blown. I couldn't see how, but you never know, sometimes things happen like that. The back story is that these had been stored after a house move, and left in storage for quite some time until the cinema room was finally ready.
I had contact with KEF regarding replacement drivers, but the originals are long since out of production, that driver model having been superseded at least twice. However, through another contact I was able to find a firm who thought they might be able to re-coil the tweeters so we could keep originality. It was during that conversation that I mentioned most of the HF loss was above about 8K. That's when the engineer said about the ferrofluid.
The process of renewing it is very much like a tweeter rebuild, so the costs are about the same. However, it gives the original driver a new lease of life.
YMMV, and maybe your speakers either aren't that old or have remained in constant use. Or maybe your receiver does indeed need recapping. But before going to that expense on a receiver that is perhaps no longer that valuable, then take time to consider measuring the frequency response of the speakers.
Incidentally, phone apps might not be the right tool to help you. Not because of any problem with the apps themselves - or at least not that I'm aware, but because some phone mics lack the HF extension required to adequately track the frequency output of a tweeter. They may give a false impression of what's going on with the tweeter.
