Thanks so much for your observation and advice.
As advised, I swapped the far ends of the speaker wires to the terminals, and surprisingly, the outcome was different.
With this swap, this time the loud, clear PINK NOISE came from the left speaker in question, not from the right.
The curret power amp is Adcom GFA-555, with preamp Mcintosh C29.
I did test this initially with Marantz amp, instead of GFA, with C29.
Another result from GFA-555, this time with GTF-400 preamp:
When in STEREO mode of GTF, the result was about the same with the GFA/C29.
In MONO mode, however, I heard about the same level of PINK NOISE from both speakers.
So, what's the issue here? No problem with the speaker itself? Any issue with the preamp?
Strange enough, I had the same result from my 4343 in excellent condition.
I applied the same test to my 4311, with C29 preamp.
You know what? The left speaker of 4343 is much weaker in PINK NOISE from the right one, just like 4343.
I'm not sure if it's the preamp or speaker issue, or any other issue.
It seems like the problem's not the 4343 speaker problem itself, as I initially suspected.