stspur - I second what Beetle says, and I'll add my personal perspective. I haven't lived with 3.6s, but I picked up a pair, thanks to this forum, which I measured and listened to and as mentioned was thoroughly impressed. Regarding upgrade in general, be very careful. Hundreds of man-hours went into selecting and voicing those particular components for the job. You could spend a lot and do worse. But, on the other hand, there are some better components available. First, let's survey what's in there.
The coils would be hard to beat with the exception of foil. But foil might require value tweaking of the circuit, since the resistance to inductance ratio is different for foil. The wire is CDA-101 aerospace grade OFHC with first-class winding.
The resistors were developed in-house and are very good within their budget constraints. But Mills MRAs are better. Safe swap. Consider doubling in the signal path.
Caps are upgradable, especially the lytics. Those VersaTronics lytics are stellar, industrial, long life caps. BUT, film is better. The "S" is indeed Solen. In the day it didn't get any better. Depending on your vintage, they are either made in France or Canada from French film, which was considered tied for best of form with a German film. Later, North American consumption was wound in Canada with same equipment and specs. Great caps. Those yellow caps are a foundation of Thielness. We found and bought a best of form German cap starting with the 2.2 in 1990. They are styrene film on Tin foil @ 1uF (usually within 1%). When the German company went out of business, we bought the remaining foil and film and contracted a California aerospace cap maker for what produced 60,000 caps. When that stash was gone, ERSE made a clone from a world-class Japanese film and foil. Eventually (we found from Beetle's late 2.4SEs) the cap was cloned in China with not necessarily lower quality.
An interesting experiment (which I haven't yet gotten to) would be to replace the 4 parallel yellow 1uF tweeter feed caps with a 4uF CSA or CMR. Clarity's ("C") copper alloy end caps are a game changer. That swap should have no circuitry implications. We would all love to hear the outcome.
As stated, electrolytics may drift over time and their sonic signature is less than best. In Thiels they are always bypassed by the yellow styrene / foil 1uF. But you could upgrade to CSA in the series feed circuits - the parallel shunts are meaningful, but less important and harder to justify for space and expenditure. The midrange feed has 2 parallel 100uF lytics. If you can fit CSA's, try it. The tweeter feed has an 8.2uF which I have targeted for a 630volt Clarity CMR.
There are various upgrade stragegies, I have 3, but most require additional real estate, which is why I am migrating to outboard crossovers. They also require the ability to tweak values, especially in the parallel shunts which are tuned to driver resonances. It would be easy to make things worse. My personal perspective is to be cautious, heed the advice from this thread, and make any changes incrementally. Swapping resistors would be a safe start. If you are a cable guy, you might consider dual inputs. And if so, and if you're my kind of crazy, you might take the woofer XO out of the cabinet and 3-4' away from the electromagnetic soup, as my 2.2 experiments have suggested.
Of course I and others here would appreciate your feedback. I gave this public response to reach more people than a PM would. But, we must beware of overwhelming those with less technical interests.
Does anybody remember the original Thiel motto? "For the Love of Music".
