Well most of the true experience of what you perceive is related to what frequency response you are receiving at your eardrum combined with any recent comparisons (psychological burn-in you have with other differently toned headphones or speakers) you've made recently. The first point about what frequency response you're receiving at your eardrum - closed back headphones can often be quite finicky about how much bass you get, determined by how good you have seated your headphone for that listening session - some are more sensitive than others - for instance I know that my NAD HP50 closed back is extremely sensitive to seating position when it comes to bass perception & I've validated that unreliability through measurements of that headphone on my miniDSP EARS - it's very variable in comparison to my other headphones in the bass, and I had to do quite a few failed measurements (which I threw out) to even get the following kind of inconsistent consistency from the HP50 (and compared against the easy to measure/ easy to wear "chuck it on" K702):
HP50:
View attachment 200141
K702:
View attachment 200140
So it's possible you're getting a different experience in the bass with each headphone seating - you'll have to experiment with that.
The other variable is that you can't always guarantee what your actual bass frequency response actually is even when you have a good seal if you haven't had your individual headphone unit measured - I've found that since I've had the miniDSP EARS and have done EQ's based on the measurements of my actual units that the bass is noticeably more reliable when listening - because I'm crafting the shape of the bass from 200Hz down to 20Hz with good precision for my unit of headphone.
The elements of "slam" and such like are all explained by the frequency response of the headphone, if you can control it or conversely play with EQ a little to enhance it then that is where you will end up at. In my experience you can get adequate slam from a headphone following the Harman Curve in the bass, as long as it is really following that curve based on your headphone's unit to unit variation & how it seals on your head. The easy option is to place a Low Shelf Filter of Q0.71 at 105Hz and increase that until you have enough bass which should hopefully introduce some "slam" back into it - however I think the shape of the bass from 200Hz down to 20Hz will dictate your slam, and it's harder to get a clear picture on what's really going on if you haven't measured your individual unit or if you suspect you have seal issues when wearing it. I don't wish to put doubt in your mind about "knowing" what your headphones are doing on your head, but headphones can be a real bitch to know what's going on with them, it's a bit of a rabbit hole, the more you know the more you realise the ways in which they can be innacurate!
EDIT: regardless of what I've said, it's true that any given headphone & EQ will sound it's best at a certain "arbitary" volume level, I normally tweak the dial to get it sound the best for any given track as long as it's not too loud.
EDIT #2: what most users can do to minimise these problems is to choose headphones with historical low unit to unit variation combined with headphones that are known to seal well to most users (this way you can guarantee with greater certainty that any EQ's you find on the net are more relevant to your unit & yourself, they will be more accurate) - that's how you mitigate those problems and get the best experience if you're not in a position to measure your unit yourself.
Interesting points, Robbo.
With respect to EQ treatment of the DCS phones to address 'slam' or 'thinness' issues, here are some things that seem to help for me. I wonder if the tweaks might be a bit timid; maybe more can be done in the mids.
1. Rather than increase bass, which seems adequate to me, I raise the lower-midrange a bit relative to the bass and treble, by decreasing the bass 1-2 dB vs. the Oratory/Harmon EQ for these phones, and by decreasing the Oratory mid/treble transition at 2150 Hz by 1-2 dB. (I.e. soften the Harmon v-shaped EQ a bit.)
2. I maintain a Dan Clark EQ bump in the 90-200 Hz range, rather than fully implementing the Harmon/Oratory EQ in that region. (Further comment below.). Perhaps because these phones have such low distortion, this doesn't seem to make for an unnatural lower-mid bloom, which I normally hate.
3. I bring female vocals that can sound somewhat distant on the DCS phones a bit closer, by ignoring the Oratory/Harmon EQ of -2 dB at1500 Hz.
4. When I'm listening for SQ, I do so at a volume that brings out the 'bloom' in these phones, as I explained in my previous post. This level is a bit higher than I'd like, but it'll do and makes the phones fun.
This story accepts EQ as one key to the 'slam' or 'thinness' issues, but doesn't focus on the more-bass = more-slam theory. It's more about which instruments sound forward and which ones sound recessed, also about the role of harmonics/resonances in the perception of the richness of sound.
With respect to the bass, these DCS phones don't ever sound bass-deficient to me. I like tight, undistorted bass that transitions smoothly from sub-bass to mids. I like the DCA phones because that is what I listen for. But I have read, and tentatively agree with, the observation that when many listeners have grown up with amplified/recorded/EQ'd/synthesized sound through speakers with their own harmonics and resonance patterns, they may be normalized to a sound that is less like live music and which has bass with heavy harmonic/resonances, closer-miked vocals, and more forward percussion. They may prefer a more close-up, speaker-like sound from dynamic drivers, even with harmonics and resonances. By contrast, planar phones may have plenty of bass, but with low distortion still sound a little more distant, less percussive, and perhaps more authentic to listeners who prefer a live-music sound.
In this context, Amir and many others have observed that the harmonics and resonances in the bass region tend to be perceived as normal sound rather than distortion. Dan Clark, in a you tube video, explains why they left an EQ bump in the 90-200 Hz region in their Stealth (or was it Noire?) phones, because low-distortion bass can be perceived as thin or inadequate in that region, even when it is not. RHO makes related comments about this FR band nearby in this thread. And for example, in this same frequency band, I often hear drum-thwacks as recessed and less 'thick-waisted' (objectivist tech term) on these planar phones than on my dynamic driver HD6XX's. More generally, my much-praised HD6XX's, with a modest EQ boost to the lower bass, constantly annoy me with a bass that sounds wrong to me, but perhaps sounds better to others (who might also appreciate the thick-waisted drum thwacks, too.) Amir found high bass distortion on the HD650/H6XX phones when the bass is EQ'd up, which he liked.
This is a little squishy and speculative. But if the 'slam' and 'thinness' issue is mostly or completely about frequency response as you tend to believe, I think we may want to look beyond bass.