What do Oil Caps do and how do they effect sound?

I don't really agree with the assumption that SS amps are 'typically' rated for higher output power (wattage) than tube amps. Yes, transistors are physically smaller than tubes, and SS amps don't require output transformers, so SS amps can often carry higher output power ratings for a given chassis size or weight. But even this construct breaks down when considering many of the top SS amps, which because they generally require more power supply capacitance and of course heat-sinking than comparably-powerful tube amps (especially as the operational bias class creeps upward), tend to feature some pretty behemoth dimensions and masses for their rated power.

Both SS and tubed amps rated for hundreds of watts of output power can be bought. Maybe the question could be better phrased, Why are there seemingly fewer lower-powered SS designs than lower-powered tube designs? Tube fans may answer, because SS amps don't approach clipping as gracefully and therefore require higher power margins to avoid unpleasantness. Hence the nostrum that tube amps can seem to 'play' about doubly powerful compared to similarly-rated SS amps. I personally don't think that this is literally true, and that power supply robustness for rated output power ultimately has more to do with the subjective sonic result - as far as rated power levels go - than does the type of output devices employed (but also that although gross tube clipping is really no more pleasant in this context than is transistor clipping, it is usually less potentially damaging to speakers when it happens).

Mostly, it just seems that in the present market manufacturers have simply moved away from lower-powered SS amps, which many did used to make, but which now apparently fail to sell well enough (with the possible exception of integrateds) to continue justifying their production. (Some of this could have a lot to do with the HT boom.) Modestly-powered tube amps, on the other hand, apparently do still sell well (and not just single-digit SET's either). Anyway, it would usually be a mistake to focus too much on rated output power when comparing amps of different types (provided none are truly underpowered for the given speakers, room, and listener). In my own experience, I have owned a tube amp rated at less than half the output power (and twice the price, to be fair) of a SS amp it replaced and vastly preferred the former, even at higher volumes (in a smallish room).

Sean: Are you implying that some PIO or FIO caps might measure better than some audiophile-grade plastic-film types? Do you have any modding or listening experience yourself that makes you tend to lean toward one or the other for certain applications?

Although Sean's post raised my eyebrows too, I think it's pretty academic to focus on how a device might theoretically continue to operate to some degree when it's damaged or broken, when obviously if it doesn't sound right we're going to want to repair or replace it ASAP.

Though I'm certainly no engineer, I think many of us realize that yes, vacuum tubes can be more robust than transistors under certain unusual conditions having little-to-nothing to do with home audio (some kinds of radiation exposure, thermal stress, higher voltages), and yes, they can be made to last a pretty long time under relatively harsh conditions where transistors are at a disadvantage (in radio transmitters, microwave magnetrons).

But when it comes to home audio, I think all we tube lovers can admit that our vacuum devices will be much more likely to fail, and anyway need a hell of a lot sooner replacement, than silicon semiconductors. In fact, tubes will audibly degrade over their useful lifetime to a much greater degree than transistors. And the ways they usually fail prematurely, such as losing their vacuum, blowing like an incandescent light bulb, or getting intolerably noisy in one way or another, are going to cause us to replace them, not marvel at how they might still be able to function a little bit. (But I would think the same goes for a failing transistor - I've just never had one go bad yet). In addition, tubes are generally more sensitive to some stuff that does find its way into the audio environment, like vibration and accidental impact.

Anyway, I've never thought "graceful" when one of my tubes died, except to the extent that it didn't take out anything around it, knock on wood (and once it did). But regardless, this whole debate is irrelevent to the question asked, and I'm not quite sure why Sean brought it up in the first place, unless he meant to imply that amps using large numbers of output tubes to develop higher powers would entail discouragingly high retubing costs...