I use both on my guitar cab. The KSM adds some cut in the highs, and PureTube fills out the warmth in the mids. Also use a SE VR2 ribbon and a Kompakt 421 and mix them differently within the tracks as needed.
Excellent choices, all!
I particularly love a 421 with guitar cabs.
I don't know if the following information is useful - you may already be doing it.
I mic up guitar cabs a bit unusually, farther back from the cab, so this may or may not be worth a try depending on taste. However it works well for me, and has for many years.
I usually set the first mic up about 12-24" away from the grille unless I'm recording a band, in which case I might still do it if I'm using a gobo.
Then I'll blend in a room mic set up at about ear level wherever in the room it sounds good.
I'm usually recording without an assistant to move mics around, so I'll use a looper, record a passage, put on headphones, and move the mics around until I find the sweet spots where everything pops into focus.
I also employ a classical stereo recording method that often works with cabs, as well as acoustic instruments: It's the Dutch Radio (N.O.S.) stereo setup. Two cardioid mics (I use small diaphragm condensers for this) set up at a 90 degree angle (45/45) with the capsules 30 cm apart. You get an incredibly realistic tone, and it's very easy to find a sweet spot.
I don't use a mic right on the grille of the cab for two reasons:
1) it can sound unnatural and overly fizzy with gain, and you don't pick up the full range speaker sound, just a fraction of what's there. And the cab itself lends something to the tone.
2) We hear cabs in the context of a room with lots of reflections; in fact, standing in front of a cab, we're probably hearing as much or more reflected sound than direct sound. When people say they can't record their amps/cabs and get a realistic sound, they might want to try miking further back.
I stumbled onto the 12-24" back idea while looking at a book with photos of 1960s Rolling Stones recording sessions. They had the mics on short stands pulled back from the cabs about that far.
From the same era, however, the photos show Beatles sessions with Telefunken and Neumann condensers much closer. There's that.
However I preferred the guitar sounds Keith and the others got and was intrigued. So I tried it, and liked it.
I realize that few people do this today, but it's my MO.