Amplifiers either work or they don't work. But

If an amplifier is working correctly, you know it, since it either works, or it doesn't work. So i am told.

Is it possible or likely for a preamp, however, to play at a diminished level, do to some small malfunction within the preamp say over time, and therefore go unnoticed by the listener, who may not be able to pick up on the gradual dimunition of the music ?
this phenomenon could be experienced within either device mentioned. I suspect you're talking about tubed equipment, where small degradations vs. time occur within tubes & is not noticed until it becomes more pronounced.
It is quite possible that a design making use of coupling caps could gradually lose output, alter tonal balance, suffer from increased noise, etc.. as the capacitors degrade and change value. It could be so gradual that you might not notice until the problem had really begun to snowball. As such, products can still "work" but that doesn't mean that they are functioning correctly or at peak performance. Sean
In reality, nearly all components can or do degrade in performance rather than or before total failure. This is true of caps, resistors, ... and solidstate devices. When things are working and sounding good, we simply take them for granted. Unfortunately, it is only when things fail that we are educated as to the many possible failure modes of even relatively simple circuitry. A good repair facility/technician is a highly valued commodity in this crazy hobby. Be nice to them because you never know when you might them!
Some amplifiers, like mine, namely my 22 year old vintage Yamaha CR220 Natural Sound receiver seems to have "off days" when it sounds either tired or distorted at higher volumes when playing CD's on my NAD C520 CD player. On other occasions, especially late at night, it will sound "normal" with full and rich sounding music with only a trace of distortion. I never noticed this phenomenon until I recently replaced my interconnects and speaker cable (after 21 years with radio shack RCA's and 16 gauge zipcord) with Audioquest Viper 's and Audioquest CV-4/Type4+. I love the clear and full sound achieved with the cable upgrades but the unpredictable amplifier can really be a nuisance. Even a quality Wireworld Electrifier power strip to plug the receiver into, with an Audioquest AC-15 power cord attached did nothing at all to improve upon the sound. Anyone else have this type of fair weather amplifier?