Custom Cans HD600/650/6XX mod: measurements and impressions Edit: This post started out in the Sennheiser HD650 Love (Appreciation Thread), which explains some of the references below. Thanks to @Tekker , @Baten and others there for the good reports that encouraged me to try this kit. ----------------- Maybe this is more appropriate for a mods thread - but there have been a few mentions of it in this one, so here it is. If staff want to move it, fine by me. Intrigued by positive mentions here and elsewhere, I ordered this kit direct from Custom Cans. Excluding VAT it was GDP 29.17 + GBP 9.60 postage to Canada. Sure, I could have just done the KISS mod, but I would have had to buy Dynamat and I didn't want to kill any spiders, so I thought I'd give this a try. The kit consists of a pair milled copper rings that fit over the spider, 4 different pairs of foam inserts to go in the magnet hole (the existing foam disc(s) under the outer spider should be removed), and 3 stick-on gaskets (a pair and a spare) that look like sections through a coronavirus to stick over spiders with legs that are too skinny to hold the rings on their own (varies between models or generations): Besides adding mass the weight is purported to smooth the airflow, the combination resulting in "more detailed bass and a little more sub-bass", which can then be tuned to taste with the different foam plugs. The foams on the left are drilled through with tapered bores; they're placed opposite ways up for the photo. The ones on the right are the least dense material and have the number 3 stamped out through them. Of the remaining pairs, the blue (3rd from left) are more dense and slightly longer than the yellow (2nd from left). Applying the mod to my 2017 (Ireland) HD6XX, TLDR is that I mostly concur with @Tekker 's description a page back in this thread: But I'm not sure about the decrease in upper bass - seemed to me to be more a change in emphasis. Measurements were made on a flat-plate coupler which unfortunately rolls off in the lower bass so doesn't show relative differences there very well. FR (no compensation) shows the tonality differences of the ring & various foams. Yellow is stock, purple (topmost) is with no foam, blue is the bored foam, green the no. 3 foam, orange the yellow foam and red (lowermost) the blue foam. SPL of the input signal was set at 90 dB @ 1 kHz for the stock configuration and was not changed for any of the others. You can see big differences in the bass, about 7 dB between no foam and blue (densest) foam around 100 Hz, and small differences in the opposite sense from 1.5 kHz ~ 4.5 kHz. Wondering what could explain the differences in bass & upper-mid/treble quality, I had a look at distortion measurements & CSDs. I didn't see anything obvious in the latter, but I'll put a subset here in case more experienced eyes than mine spot something. Stock, no foam, drilled foam, densest foam: Distortion seemed more interesting. Care is needed in the interpretation because of the different SPLs in the bass, but comparing measurements it's possible to see changes particularly in how the 3rd harmonic behaves. Ignore the big spikes in the bass, they're because of electrical noise nearby in my building. Let your eye follow the baselines and note how with more foam H3 is higher in the bass & lower mids compared to the other harmonics: Now just comparing the 3rd harmonics of all measurements - same colour scheme as the FR graph: Notice between about 40 & 200 Hz how the tan, green and orange curves (stock, '3' foam, yellow foam, respectively) are about the same but purple and blue (no foam, bored foam) are lower; and how the red curve (blue, densest foam) is higher from the bass up to about 400 Hz despite being lowest in level through there on the sine sweep (FR graph). Notice also the higher and increasing level of the stock (tan) curve above ~500 Hz. These curves represent the levels of the 3rd harmonic at the excitation frequency, so would manifest at ~120~600 and >~1500 Hz, respectively. However, they're all below -50 dB down from the fundamental level, so it's hard to argue that a few dB of difference down there would be audible. But maybe they cause or correlate with some other effect to generate the perception of more textured, better quality bass (don't know about the blue foam - as the FR shows bass is kind of absent with that one) and smoother upper mids & treble. My favourite configuration: I go between the bored foam and the '3' foam; '3' seems tonally the most balanced but bored seems timbrally the most correct across the range (and sometimes I don't mind a bit of extra bass).