life span/failure rate of filter capacitors?

Note that loss of capacitance would result in poorer filtration and eventually an increase in your noise floor.

Nelson Pass is one of my audio idols, but I'm not sure his words are entirely relevant to the OP.  Cap lifetime may not be the same in tube & ss amps.  The energy is stored at a much higher voltage for tubes, ss power supplies hold a lot more charge.

Not all electrolytic caps are created equal. Some have high temperature ratings (good), higher mtf (good), higher ripple current (good), higher esr (bad). Thing is, each one of these parameters can be important, so try to find out everything you can about the caps you have, and specify a cap that is at least as good in every respect.

I buy all my electroytics from Digikey (no affiliation). Great company, tremendous inventory, next day delivery. And no surprises.

(And I assume that the filter caps are the large electrolytic caps with the screw in terminals?)

I'm pretty sure you are speaking of power caps. Filter caps are the small uF units that filter out unwanted signal.  The big (blue?) ones with threaded connectors are your high voltage supply for the valves. 

Power caps are not as critical to sound as the filter caps. 

For me, I prefer to buy from the manufacturer as it assures me of the correct part and quality. It saves me from errors and gives me recourse if there is a problem with the component. Caps are cheap compared to other amp parts like valves, transformers, etc. so even if you pay a higher % markup from the mfg. it's still a small $ number. 

@immatthewj  - if the amp is idle then those years typically do not count but it all depends on how much is was run for the 14 years.

Like I mentioned above, you can use a larger value but make sure they fit.  You can call Chris Johnson at Parts Connexion and ask his advice as he was the manufacturer of Sonic Frontiers products back in the day.

Actually power caps are not as critical but they do have an impact on sound especially on bass response and clarity.

Happy Listening.

@audioman58   IMO capacitor dry out powered or not.  Unpowered capacitors are likely at lower temperature drying slower.   Electrolyte tends to eat out dielectric (aluminum oxide) reducing breakdown voltage, while presence of the voltage rebuilds this layer.  Gear (or caps in storage) unpowered for many years (likely >5), should be powered at lower voltage (like half) increasing it slowly (Variac) over at least a day.  Unfortunately SMPS often don't work at all below certain voltage and output capacitors cannot be fixed that way.