filter cap failure - gradual or sudden?

[Groover]

can old filter caps be sufficiently deteriorated to cause an amp to sound bad (specifically mushy/warbly low end) but still good enough that the amp does not hum?

amp in question is my '70s Vibro Champ and if I'm reading the date codes correctly the cap can is original. the amp has no noticeable hum

I'm trying to establish whether new filter caps will tighten up the low end

[Matthias]

Got the saggy bottom?
More uF's would tighten up the low end...
Bad filtering could cause a warble.
If the old ones are original, might as well change 'em out anyway.
There is always the possibility that someone may have gutted the originals and stuffed 'em with new ones....but unlikely, unless you got it from a collector.

[Enzo]

In a typical tube amp, the first B+ filter node serves the output plates, the second the screens, the next couple the preamp circuits. The first filter removes most of the hum, ripple we call it. By the screen node, the B+ should be quite clean. The remaining node filter caps are more for decoupling the stages than reducing hum, because at those stages there really isn;t any hum on the B+. If those caps dry out, you lose decoupling, so stages can interact. Depending upon phase relationships, that can mean even oscillation or it can mean degradation of sound.

Even the main filters will still filter when drying out. SO a 40uf cap acts more like it was a 5uf cap... sorta. The thing can still clean up the B+, but under loads it cannot hold up and you get distortions. SO in answer to your first question, yes.