Advice Needed On Recapping Power Amp

Going to higher capacitance means also higher inductance hence slower response to rapid current demand.   It will also reduce ripple and that might not be a good thing, since very low ripple, especially when combined with low ESR means very high and very narrow current spikes charging capacitors and that might couple to any inductance in the rest of the circuit.  I would agree with  kosst_amojan and would not try on my own to redesign an amp.  In some cases it might work in other it could make it worse (or damage rectifier).

Bypassing slow large inductive caps with fast low ESR cap might be not the best idea, since it creates parallel resonance circuit that might ring.  Better option would be to get low inductance electrolytic caps (like slit foil type), but they are expensive.  Better yet is not to "redesign" something that works just fine.  There is also an issue of reselling such "improved" amp.

15 year old capacitors, unless they are in very warm place, should be good for another decade.

Kijanki,
    I've never claimed to be the sharpest tool in the shed, but as said above, I've done this many times. I have only had positive results.  How do propose that you get parasitic inductance introduced in a properly designed power supply?  The only issue that I can see is a poor excuse of rectifier. As long as you bridge is rated for the current you should be fine and An ideal cap has zero inductance or resistance on its own. 

@rlb61     after all of this, I'm sure that you'll decide not to recap, but as we have all state, not really needed at this point. 

Timlub, Things that look simple not always are.  There is no such thing as "Linear Power Supply", unless Linear means "Unregulated".   In reality it is primitive unregulated switcher that operates at 120Hz and switches at max voltage.  Current from mains is delivered to capacitors in very narrow current spikes of very high amplitude. Amplitude of these current spikes depends on transformer, power cable, ESR of capacitor etc. while width of the spikes depends on the ripple amount that, in turn, depends on the load current (definition of the switcher).  In addition to current spikes charging capacitors you get also very narrow spikes at the peak of the (rectified sine wave) voltage when rectifier starts conducting for a moment in opposite direction to quickly snap back.  To widen this spike there are "soft" diodes (like HexFET), that conduct fast but snap back slow.  I'm trying to show that operation of such primitive supply is not as simple as it seems and the fact that larger cap works means only that safety margin for rectifiers still exists but was reduced and might be very small.  If we believe that designer put unnecessary margin by mistake that's OK, but I would ask him  :)

As for inductance - electrolytic caps are inductive.  Inductance of such cap is proportional to capacitance.  Increase in capacitance means increase in inductance.   Believe it or not, but power supply caps are in series with your speaker (circuit is closing thru them) and their inductance can affect response time.

@kijanki     Not to be argumentative, this is more for me to be informed. First, I've never added capacitance to a switching supply and This is just not how I understand inductance in relation to a power supply cap. 
Inductance would be creating a magnetic field, yet As the capacitor charges, the magnetic field does not remain static. This results in electromagnetic waves which radiate energy away. So the magnetic field dissipates. The cap itself is not inductive. 
So, I can see faulty, weak or under rated rectifiers.  Maybe if you flipped the switch at the exact instance when your ac was hitting its peak in the sine wave. 
Again,  I've done this a bunch.... The first time was in 1980.  Parasound built a chipped amp around a Sanken STK084... I called Dick Schram and he sent me 2 circuit boards, I ended up using an STK086, put higher grade parts in it.  His amp had 4 - 6800 mfd caps in it.  I used a separate supply case with 12 - 6800mfd caps.  That was my first foray into increasing capacitance and wow, what a difference that it made.