Is there an average life span for a preamp?

Filter capacitors will hold up better if they continue to get used on a regular basis. However they will still degrade. Any piece that is over 20 years old should be considered suspect. It may well still work, but that is not the same as saying that it is working as well as it should.

Atmasphere, My calculation based on Nichicon data shows that at 35degC (95degF) standard caps should have 80% of capacitance after 17.8 years. Each 10degC cuts life in half, so I would estimate not more than 10 years for caps in hotter amps unless 105degC or long life caps are used. When ESR of the cap increases to cause substantial heating from ripple current they get even higher ESR (since ESR increases with temperature) and thermal runaway causing to blow fuse or scored top. It happens when capacitor is very old and starts running hot. It should be easy to detect such caps especially since there should be other symptoms like less dynamic sound, poorly defined bass, 120Hz noise etc.

I can understand that many audiophiles have "gardener's syndrome" (constant need to trim and re-pot) but if it sounds good to you - don't touch it.

Kijanki, that sounds about right to me. However as a side hobby I like to restore vintage analog synthesizers. Its pretty well universally accepted amongst those who do such work that the filter caps in the power supply should be among the first things to be changed during a restoration. Of course, many analog synths can be a few years older- some are from the late 1960s. But my Prophet 5 (http://www.vintagesynth.com/sci/p5.php) is from the 80s and there was no question that new filter caps helped it out- it sounded better and was far less sensitive to line voltage after that!

"universally accepted amongst those who do such work that the filter caps in the power supply should be among the first things to be changed during a restoration"

I agree. I would also like to have caps changed during restoration of old amp but would not do that otherwise (without a cause).

I assume that amps with minimal feedback are less forgiving but issue of increase output impedance (ESR of capacitor) is complicated one. My class D amp with DF=4000 at low frequencies sounds great with my speakers. I'm pretty sure that increased output impedance might make it sound better with other speakers. In such case audiophile satisfied with great sounding older amp replaces caps, following advise of an "expert", for the worse sound. Do I make sens?

Kijanki, there may be something I'm missing but I would not expect much change to the output impedance with most amplifiers or preamps on account of the condition of the filter caps. In every case I have seen if the caps are ailing, replacement universally has the unit sounding more musical.

No speaker is so sensitive to damping factor such that you would be able to hear a difference between 4000 and say 200. The damping factor does not play much of a role with any speaker unless it falls below 20 or so. But as you say, some speakers *prefer* the damping of the amp to be lower.