Cary SLP98L upgrades--yea or nay?

@northman yes, your list +Grayhill switch selector.

These upgrades and different tubes can yield good results in different ways. Having tried rare vintage NOS quads, new re-issues, and some of the new boutique 6SN7s, the sound will change, and more so with good caps installed in your SLP-98L. 

Easily reversible upgrades, such as trying different tubes would be the best way to start your upgrade path.  If ANY upgrade changes the sound, which it should, that change can be for the better or worse, depending on your system and your taste.  that is why something like trying different tubes is the best approach.  There are many vintage tubes to try.  While it is hard to verify that any old tube is truly new old stock (never been run except to test), those that test strong tend to  be quite long lasting anyway.

The effect of any capacitor change is much harder to predict.  Even when really good capacitors replace lower cost, supposedly inferior capacitors, the result may not be an improvement; good designers voice their components with particular parts choices.  Many years ago, the "hot" capacitor, particularly for coupling capacitors, were "Blackgate" capacitors.  In the right circuit, they are indeed very good and worth their steep price.  I heard an amp from a custom builder that the builder asked me to listen and give my opinion.  I was quite reluctant to inform him that this amp was WAY off of his best efforts.  He laughed because it was one of his amps that had been brought in to undo the "upgrades" that another company did.  These upgrades included Blackgates and other fancy parts like Vishay foil resistors.  The same builder did use Blackgates in some of his amps where such sounded right.  In one amp, the buyer complained that the amp did not sound as good as it did in the builder's shop.  The builder went to the buyer's home and heard it and agreed that it did not sound that good.  They went to get some coffee and when they got back, the amp, which had been on all that time sounded great.  It turned out that the buyer, a very busy surgeon, only listened in short intervals so he did not hear the amp fully warmed up.  The builder replaced the Blackgates with something else that warmed up quicker.  Parts choice and design can be quite tricky.

Thank you both for your thoughtful, helpful answers. I haven’t done tube rolling in years (ie decades) but I’m looking forward to having a go.

@larryi , everything you say makes sense. Since there seems to be near-universal enthusiasm about the F1 upgrades, I was hoping that I’d find a similar no-brainer response about the Mundorfs. Still, I’m pretty sure I’ll go ahead with it and hope for the best, though starting with tubes is a fun and immediate prospect.

@decooney , thank you. I've read some of your posts on tube rolling with the SLP 98 and they're a great help!

I’ve listened to a lot of Cary SLP-98 preamp’s with various mods.  To me in its original form it’s a bit lackluster. Not the most extended highs, nor most extended lows, and not the most detail or soundstage compared to more expensive, preamps, but a really nice fluid, sweet sound.  Definitely a preamp worth upgrading.  The F1 is a big step up. But of course, everybody has their opinion of what is the best capacitor. The big problem is what capacitors will fit in that preamp chassis space.  I happen to really like the Mundorf’s (at least they’re best) and thought they were excellent and a big step up from any of the other capacitors that Cary Audio tried. Are there better capacitors? Well of course, but what will fit? My favorite of course is the V-Cap CUTF and you might find some way to fit the .22uf in somewhere on a 98L but you’ll never get the 5uf output coupling caps in there.  The Mundorf oil caps are a little bit more detailed, but some may feel could be brighter. That’s going to be very dependent on the rest of their system though. Meaning if your source or amp is very bright, overly detailed it’ll show up.

I do like the Hex-Fred’s and recommend them to give more speed, detail, imaging and improve bass speed.  I have rarely known anybody not to like them, but there are people who swear against them for technical reasons. 

Kimber Kable Will improve the sound quality, and yes it does like any good high-grade copper cable will. The problem with Kimber is there is no shield and consequently, if you’re playing another source on another input, you’ll hear it come through. Which of course you should be turning the other source off, but people will complain about that. I prefer something with a shield and recommend wires like something of a OCC quality.  
Also the Greyhill selector switch is good and provides higher quality contacts for switching inputs. It’s cheap enough that I would definitely just have it installed. 
That my two bits. 

I own a Cary SLP-98p and I fully agree with the comments from larry, russ and harpo.  Sweet, musical, but a bit "sluggish" and thick-sounding.  Nonetheless, you're wise to keep it unless you can afford something a lot better.  (I myself moved to a Don Sachs-style 6SN7 Aikido preamp of my own construction, which retains the wonderful linearity of the 6SN7 but is much faster and more transparent).  I think the upgrade would be worth it, since anything better would cost at least $3000.  I would go with the oil caps.  I don't think you'll find them dry or bright.  I use Audyn Coppers in my preamp, but unless you find someone willing to offer you a choice of output caps, you're kind of stuck with Cary's recommendation. ;-)