I have joined the club! Ha!
On the esteemed advice of (guess who...? ) I popped some 0.01uF AudioCaps across the factory caps in the 3103.
Simple enough, and using the hemostats as heat-sink proved perfect. But then I got to wondering how come we all HAVE hemostats, hey? I know I used to have more pair then I do now, and that my sons are likely relishing them like I did when I pinched MY dad's!
Anyway, here's the report: I only did one of the 4301B pair, and A/B'd them, swapping cabinets side-for-side, and using L+R, stereo, and reverse. I sat between them, and both near and far field.
The difference is subtle, but noticeable, I think. My impression is the bypass caps "open-up" the sound. It is somehow not as "crowded". It's like the highs in the non cabinet have a subtle HF "hair" around them - hard to explain. Sort of a subtle harshness too, that would otherwise not be noticeable, but is absent in the bypass caps network. Certainly, the attack and transients seem much more clean in the bypass caps cabinet - the bypass cap network has a real precision and sharpness to attacks, but without any harshness around the sound. The LF seemed more open, and full, overall.
¿Am I making sense?
I tested a variety of source material, but the most effective for this proved to be: Skip, Hop & Wobble by Jerry Douglas, Russ Barenberg and Edgar Meyer, ±Sam Bush.