Why does my AVR volume show strange numbers like -25 dB, and why must I turn it so high?

The dB scale on your AVR is not "how loud". It is "how far below reference", so needing minus 20 for a satisfying volume is completely normal and says nothing about a weak amplifier.

Here is what the number means. After calibration, 0 dB on most AVRs corresponds to cinema reference level: the level the film was mixed and monitored at, which is loud enough that few people run it at home. Minus 25 means 25 dB below that reference. Home listening lives anywhere between roughly minus 40 and minus 15 depending on room size, speaker sensitivity, source level and taste. Serials at minus 35, a film on Saturday at minus 18: both ordinary.

The scale reads strangely because it is a relative scale, but it is more useful than 0 to 100 once you get used to it. Minus 20 means the same thing after every calibration, whatever the speakers. "Volume 47" means nothing to anyone.

Two caveats are real. First, if you are pushing towards 0 just to reach normal loudness, something is off. Check speaker sensitivity (a low-sensitivity pair asks the amp for a lot more), check the channel levels have not been trimmed down heavily by auto-calibration, and check whether a source outputs an unusually low signal. Some TV apps and some streaming boxes do, and you end up compensating at the AVR for the app's timidity.

Second, distortion at high settings is your real limit, not the number on the display. Clean loudness at minus 10 is fine. Strained, hard, compressed loudness at minus 20 means the system is being asked for more than it has, whatever the display says. Your ears and the speakers' excursion are the meter that matters.

Where does the rule stop mattering? Regularly near the top of the dial in a big room, with the sound just about adequate. That is a sizing conversation, not a settings one: the room, the seating distance and the speaker sensitivity are asking for more amplifier or more speaker than you have. Tell us the room size, seating distance and the models and I will tell you which of the two.