A quick update:
I've been playing with the L-pads and took some SPL meter measurements - nothing super-detailed or precise, just at the 10 octaves from 32Hz to 16kHz, doing a few runs to ensure the readings came up the same each time. Some of the readings surprised me, while others confirmed my listening impressions:
- In "normal" mode - which produces less low bass than "extended" mode - these suckers are F0 at 32Hz; in other words their SPL at 32Hz is the same as their SPL in most of the rest of the frequency range. Pretty nice! I tried enabling extended mode, but it either made no difference or added a little too much in-room bass (my comparison was sighted, so I don't know if my perception of too much bass was real or confirmation bias).
- There's a small increase in energy at 63Hz and a somewhat larger one at 125Hz. This was a surprise to me, as the speakers don't sound boomy to me, and I'm pretty sensitive to elevation in the 125-200Hz range. I am guessing these are room modes, and I will be very curious to find out what their Q is, becuase I'm guessing it's pretty high aka these modes are fairly narrow in frequency range, which is probably why they're not jumping out at me.
- There's a few dB elevation at 2 and 4kHz, which was not a surprise at all, and in fact kind of a relief, because this was exactly the range where I was sure I was hearing something that wasn't quite right. The original polydome mids disintegrated over the decades (super common problem with these), and my father replaced them with fabric domes that are a little more sensitive, so the L-pads are incapable of attenuating these replacements enough to get them to be flat with the other drivers.
So with this info in-hand, I've gone ahead and ordered a MiniDSP SHD, and when that arrives I'll put it between my Oppo 205 and my amp, and let Dirac take care of everything.
In the meantime, I've enabled the 10-band EQ built in to the Apple Music app on my Mac mini that I use as my music streamer, and dialed in 2-3dB cuts at 125Hz, 2k, and 4k. This is just a stop-gap until the SHD arrives, as it's a crude, incomplete solution, and it doesn't allow me to have the EQ when I play physical discs from the Oppo. But even this simple EQ'ing seems to have addressed the issues I was hearing, so I would guess the sound will only improve further after Dirac does a more fine-grained and extensive removal of nonlinearities that are influencing the sound now but which are not jumping out at me.
So I think once the SHD arrives and I get it all set, I'll be very happy with this setup and I could be set for the foreseeable future.
If after a couple of months there is anything that bothers me about them, though, at that point I'll feel satisfied that I've done the best I could do with them, and I'll sell them and get a pair of Genelec 8351s. But as of now I'm feeling pretty optimistic that these Kappas are going to suit me nicely.