[greg7]

Since I'm going for a very vintage sound (my preference), what caps have you tried (speakers or coupling) that struck you as super soft and/or smooth sounding (which would include the effect of being very low distortion). Resolution's irrelevant here. I need large values (2.2uF and 3.3uF), so Russian jobbies are out (already tried them years ago -- they were "OK")

[Bendingwave]

Jantzen audio Z silver.

[walkern]

I found the Jantzen Z Superior caps quite smooth sounding in a DAC that needed a little smoothing out.  They didn't  roll off the top end like some other caps.  Extension, frequency balance and resolution were fine.  Sounded to me like just less hash and grit and spit.

[richidoo]

I too love red Jantzens but I wouldnt call them soft or smooth. They are as detailed and lively as it gets before crossing the hifi Rubicon and getting overstimulated by detail. I use them in Xo with a very detailed dimpled silk dome tweeter and I love the gentle but detailed treble compared to my metal AMT tweeters on my other speakers.

If you really want to smooth out the transients and throw away some detail go direct to the source of detail in the speaker. That is damping. Damping = detail. You can't easily change the mechanical damping of the drivers but it is very easy to reduce electrical damping by increasing resistance of the load. The much maligned "Damping factor" is the ratio of source impedance to load impedance. Your amp provides a set source impedance and this drives the varying load impedance of the speakers. Damping factor increases when amp impedance drops or speaker impedance rises. Solid state amps output impedance is typically <0.1 ohms while tube amps typically >1ohm output impedance. So tube amps typically sound less detailed with less slam than SS. Exceptions are due to damping factor.

You can't easily change the amp's output impedance but you can add resistance to the load by using higher resistance cables or even adding a small value resistor of adequate power rating in series with the positive SC lead. A one ohm 50w resistor will smooth out the sound nicely, top to bottom.

The resistor power rating is determined by amplifier power rating, speaker sensitivity and how loud you listen. A 50w amp on 90dB speakers playing compressed music like rock jazz only needs 10w resistor.  mills 12w wirewound would be a great choice.
If you have power hungry speakers and 500w amp and you use all of it then a 50-100w resistor would be about right. To test the concept any 5-10w resistor could be recruited temporarily. If you like the effect then you can adjust the resistor value to taste. Probably more than 0.1 ohms and less than 2 ohms. A 2 ohm resistor will make it sound like a SET amp from 1930s. Too much smoothing for me but it might be just what you're looking for and that's not attainable with caps.  Nice thing is you can experiment and change it over time.

Alternatively you can do what many audiophiles do by using cables as tone control and choose smoother sounding cables that kill off electrical damping by various means. Lamp cord aka zip cord is probably the cheapest and among the best at reducing detail but there are better sounding choices.

[Rusty Jefferson]

....Resolution's irrelevant here. I need large values (2.2uF and 3.3uF), so Russian jobbies are out (already tried them years ago -- they were "OK")

I'm not sure what 'Russian jobbies' are but the Russian KZK polypropylene capacitors fit your description of what you're after.  I have them in both coupling capacitor (tube amplifier) and crossover capacitor (horn speakers) positions. They are quite smooth. Have you seen the Humble Hi-Fi capacitor descriptions? You'll see the KZK listed there with many others as well.  Looks like they could be harder to get right now do to the Russian war, but I'm sure they could be had.

https://www.humblehomemadehifi.com/Cap.html

[Bendingwave]

I've tried both the Jantzen Z Superior and the Jantzen Z Silver (both@5.6uf) and the Silver is smoother then the Superior.

[FullRangeMan]

In my limited experience the Russian caps are cheap and reliable, they were used by Lampizator GM70 amp due tech reasons, I think they are a very nice standard to compare others brands.

[Folsom]

Russian PIO's are available in a pretty wide range of capacitances.

[Mike B.]

I would also be looking a PIO caps. Check out the Jensen brand

[DaveC113]

Yup, those big metal-case, oil-filled caps are likely what you want.

[Scroof Neachy]

the ones that take 400 hours to break in

[FullRangeMan]

I was told by an tech that that these metal can caps are log life, not sure how many years would be that.

[I.Greyhound Fan]

Read this-

https://www.humblehomemadehifi.com/Cap.html

http://enjoythemusic.com/diy/0708/capacitor1.htm

http://www.laventure.net/tourist/caps.htm

https://www.head-fi.org/threads/orgy-of-capacitors-the-cap-thread.284863/

Take a look at obbilgato Gold caps.  Very smooth and holographic in a Pass preamp that I built.  They have a new Obbligato Gold Ultra premium cap that is supposed to be smooth.  The original golds were soft in bass punch but what was there was warm, musical and smooth.  The sound stage was huge and 3D. I replaced them with Clarity caps that had much more bass punch but less of a holographic sound.  I would try the Obbligato's in a tweeter circuit.

https://www.hificollective.co.uk/components/obbligato_gold.html

I have bought from them before and shipping to the U.S. is quick.

My son and I just modded the crossover in a pair of old PSB B6 speakers for fun.  We replaced all the electrolytics with Clarity CSA's and the sand cast resistors with Mills.  The Clarity's are smooth and lean to the ever so slightly  warmer and darker side.  They improved the texture and clarity.  They worked well with the mid-bass driver, but it sucked the life out of the tweeter.  There are 2 caps in the tweeter circuit.  So, I replaced one of the Clarity's with a Dayton Audio Ultra precision cap and the treble went from being rolled to very detailed with a huge sound stage and great transparency.  They have a little bit of sibilance, but they only have 10 hours on them, so the jury is still out on them.  If the sibilance does not improve, we are going to try some Jantzen's or the Obbligato's. Currently the speakers sound much better than stock.