I'm kind of a capacitor nut, have been for years from upgrading audiophile and studio and guitar gear. I go thru phases where I prefer a certain sound, and then gravitate to another. Super clean tube preamps have been my specialty as I've been making them for pedal steelers, and in that super-clean, high-voltage environment, cap's really reveal their character quite a bit. Some basics are that true film & foil cap's have a distinctly 3D character to them. Foil just seems to do that more than metallized film for some reason. Tin foil sounds warmer and meatier than aluminum foil, yet still with all the top end detail and refinement. Polypropylene film is more "hi-fi" than mylar (polyester). Mylar has a "juicy" or sweet color to it generally.
All cap's break in, especially the ones with high voltage on them. Also the cap's with higher voltage on them reveal their character more.
Real small value cap's are gonna be either ceramic, silver-mica, or polystyrene. Ceramic has a hard "glare" and graininess to it which is sometimes good, often bad. Silver mica is just super clean and fast. Some say it sounds too hi-fi. In a clean preamp I tend to much prefer it over ceramic. It's less fatiguing. Polystyrene is perhaps the silkiest and highest performance cap in this bunch. I really dig them as Fender "treble" cap's for that 250pf position.
All this said, I tend to gravitate toward a combination of cap's in the overall signal path with out letting one particular flavor dominate too much. Early in the signal paths I lean toward film & foil cap's. Further down the pike I like what metallized mylar does. In the tone stack I've been into using the 225p series Orange Drops. These are the same cap as the vintage Fender "blue molded" types, still made by Sprague in the US. These are a mylar film and foil cap, and they are sweeter and less harsh than the more common 716p or 715p series orange drop. I like the Mallory 150 mylars for outputs. I'm sure some of these opinions will change over time, just as us guitar players go from pedal to pedal now and then.
For the new SMS Classic pre, I'll be open to custom cap selections. I know some guys will want an all paper-in-oil cap signal path. It is expensive, but man that also is a glorious sound once they cap's have broken in for 100+ hours. Some guys may want all Zoso's, or all Jupiters, etc. I can go with the cap's from the famous Babb's amp photos which many believe was set up for Jerry's use. That's a combo of blue moldeds, some orange drops, a ceramic treble cap, a mica HF boost cap, etc. Overall these cap choices are not huge factors, but for any of us with ears, the more we hone in on something, the more these little factors actually become quite noticeable. To me I tend to notice the depth or complexity of a 2-note harmony or a 3-note chord. Different cap's will enhance certain aspects of the harmonic spectrum there. Then, of course, the very high end treble, how spikey, how silky, how round, how glassy, how gritty, how juicy, etc. The top end is really a critical zone when playing clean. It can hurt or it can heal. And for each player, the top end reaction in the preamp has a lot to do with pick and picking style, as well as the amp and speakers that follow.
To me, none of the commercially available tube preamps have these finer details nailed, nor really give a crap about them. Most preamp designers are going for dirt, and the clean channels are just run of the mill in their approach. And I don't think any of them are as obsessed with Jerry as we are over here. So hopefully this new preamp is gonna be a real useful tone tool for a bunch of us, me included.
... and it's just like any other day that's ever been...