phono preamp capacitor upgrades??

My Krell ancient KBL just went in for service a few days ago, Krell advised me that the fix was likely some replacement caps (not sure which ones). I've also got a Threshold FET 10/pc phono preamp and it's pretty old too - at least 12 years and it's on constantly.

Would I do well to replace/upgrade capacitors? If so which ones, Electrolytics or all of them? And how to decide? It seems like lots of the audiophile capacitors are incredibly expensive, say $15 for a single capacitor and there are about 30 in this thing - 16 electrolytics and about the same number of metalized film (?) caps. So this is a lot of $$. Any way to prioritize? I don't have a schematic, so it's tedious to reverse engineer which is which.

Any advice or suggestions on how to proceed (if at all)? Thanks,

Power supply electrolytics can lose their ability to hold a charge over a period of time, especially if your equipment is not used for a long period of time. They will probably last about ten years or so. They can be reconditioned but it's usually fairly cheap to replace them. You nearly always must use electrolytics for power supply coupling caps. This is especially true for tube equipment to block D.C. The other types will last almost indefinitly although some of the newer caps may produce somewhat better sound if in the signal path.
Any electrolytics in the unit - especially the ones in the signal path (not all equipment has electrolytics in the signal path) tend to become leaky over time. You can often hear the effects of leaky caps in units that have a "crackly" volume control (that's due to DC leakage).

One can upgrade almost any unit that has electrolytics by simply replacing them with new units, for best results use the ones that are "audio grade" which have somewhat better specs than the usual electrolytics. Mouser sells these things.

In general, if there is room, higher voltage rating units are a good idea.

Film caps rarely break down. They don't need changing, except to upgrade from mylar film to polypropylene IF it is possible to find the space for the generally large polyprops
in the same space (usually you can't).

Ceramic caps can often be replaced by film, especially polystyrene (easiest in low voltage circuits). This is a good idea, since ceramic caps are not among the best for audio applications. Watch your voltage ratings here, if you're in a tube circuit.

Again, the film caps usually never need replacement, the electrolytics on a PCB are generally cheap. The cost is in the labor for doing the swaps, not the components in this size. Expensive polyprop film caps are usually large ones in crossovers, not the small value ones found on PCBs...

If there is no malfunction in your preamp, there is nothing to fix or change. If the unit is run regularly, the electrolytics are likely to last 20 years or more.

Hope this explains the situation,

Of course you should never use polypropylene capacitors in a power supply. They just won't hold up.