Speaker crossover mod. Your advice?

Yup; done this sort of thing often, most recently in a pair of Audio Physic Avanti IIIs. In those, the 2 caps in series with the tweeter were a 6.8uF and a 15uF. I replaced both with the highest-quality-composite cap I chose to afford, and this time I wasn't cheap. Altho the highest-value SoniCap Platinum (Teflon-film) cap Sonic Craft lists on the site is a 4uF, Glowacki had a pair of preproduction 6.8s, so I bought and installed them, and the 15uF turned into a 12uF SoniCap Gen.1 'propylene plus a 3uF Platinum. (Claity Caps were original.) Altho I'm no golden-eared audiofile the way some of my friends are, I heard MUCH-cleaner and less-edgy treble, so much so that I was able to increase the average level of listenable treble several dB. I suggest a combination such as I used or perhaps with as high a value of MultiCap RTX you choose to afford with a SoniCap Gen.1 to fill the value to the original.

What is/are the value(s) of the tweeter cap(s)?

BTW, then it's the midrange's turn. :-)
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Modifying crossovers can be very rewarding sonically. I'd suggest doing all the caps and don't forget to do the resistors.

As for what caps to use, there are many good choices to fit your wallet. Sonicap Gen I's are very good and reasonably priced. Clarity MR's are another good choice, but cost a lot more. I do not recommend Solen.

For resistors, Mills are most folks go to.

Pitfalls....make sure you have the room for new caps as most of them are going to be much larger than stock.

Some good reading, www.humblehomemadehifi dot com/Cap.html

considering you will not make any incorect wiring after upgrade its very good idea- focal react to this upgrades very good. change caps in tweeter section ad resistor in tweeter atenuation section. if yo like to play speakers loud also change bass inductors to transformer types, or transformer foil inductors. just keep exact RDC