Alsa/jack audio : Confused about alsamixer capture levels generating hard clipping

Not sure if the topic is too audio-specific to be posted here, if so then do tell and I'd appreciate suggestions on proper forums. My LP recording setup is through an M-audio 2496 card on linux running jack, using a command line script to capture. I use the hardware input to capture and set the recording level using alsamixer; My usual capture level has been -3 dB for years. These days I'm testing a new phono preamp that has an higher output level than my other units so I lowered the capture level to -6dB but ran into clipping issues. Investigating this problem, it turns out that setting the capture level below -3 dB on hardware channels "H/W Multi" (left) and "H/W Multi 1" (right) clips the signal ! Perhaps some pics would help illustrate the phenomenon. Below are shots of the same high modulation passage of an LP record at different capture levels in alsamixer. Note that except for the 0 dB test, there were no Xruns or overruns during the recording process. At 0 dB I get overload peaks as can be expected; at -3 dB things seem ok like my usual setup; Note the recorded amplitude level. But as you can see, lowering capture level further not only reduces overall recorded amplitude but clips the peaks, even if they reach a lower amplitude than the recording at -3 dB. I always assumed the alsamixer capture level worked as an equivalent of the "recording level" on the cassette decks of my youth, e.g. simply attenuated the signal. Now it seems it both attenuates and limits somehow.... I'm confused! Please help me better understand what's going on here and thanks in advance for any insights. -Joe