this one is interesting to me - i've heard a fair number of people comment on the BF2 vs the slew of well-measuring chi-fi dacs like topping, SMSL, etc. and quite a few people call out the BF2 as having much better macrodynamics. i'm not too sure myself since i currently only have the BF2 on hand but i thought it was interesting you had a different experience (not that it's wrong or anything!)
incidentally, where did you put the sorbothane on your sigma?
Yeah, I was expecting to hear something artificially exaggerated based on a lot of my past readings. A few months ago I exchanged some DMs on here with someone who said they found great synergy with the D90 and Stax, that the D90 gave a big and holographic soundstage to the Stax, and that the Stax was able to properly execute the high-end coming from the D90 (some have said the D90 is thin and can be piercing up high). I love my BF2, but I wanted to test what the polar opposite of it's hefty, warm signature and hear this holographic soundstage. I have always gravitated towards warm, wet thick sound sigs, but when I first heard a Lambda NB on a SRD7 with a cheap car stereo amp, I was hooked. I had been hiding from the treble, but the way in which the Stax so effortlessly and beautifully presented it, like a prima ballerina, opened my world from Nighthawks/EMU Teaks/Argons (and even Sundara/LCD2C tbh) to the fluid, fast, accurate and musical nature of Stax. RR1 and Hives didn't do it either - so it wasn't elecrostats, it was Stax. for me. That first experience was with a Modius, and a Khadas Toneboard, for reference.
Anyways, I digress. One of my concerns as I upgraded from the SRD7 and SRD-X to the T1S/006t along with the BF2 (and a Jolida Glass) was that I was layering tubes on top of tubes and covering over the effervescent shimmery sparkly sprites that I loved so much. So I wanted to trial the D90 to either validate or eliminate that. I'm sure that the D90 is doing some extrapolating and processing, and I hate having to use complicated UIs like the D90's, but my experience has been that the D90 has a wider and taller soundstage, and does a better job of fighting through the tubes to force the details out. The BF2 is like a wall or a wave of wonderful sound, the D90 is like a rainstorm of sound (?), where you can more clearly hear and feel the different placements.
On the downside, I detect a slight unnaturalness with the D90. Not one I'd hear without being able to immediately A/B between them, but the BF2 had the edge there. Yesterday I listened to both the D90 and BF2 with the 404 Sigmas, and due to the nature of the housing, the sparkle is muted and some of the high end details are too - but the soundstage was still larger on the D90, to me. I think DesignTaylor referenced listening to Stax as less like you're lounging in a chair, and more like you're sitting up, actively 'listening' to the music. Stax can reach out and grab your attention. I personally have never had an issue just zoning out while listening to Stax, but I understand the analogy and I think the D90 does a better job at that sitting-up, focused listening where the BF2 is for the leather chair-and-whiskey time. And I think my main caveat to what I'm saying is if you have a 252/353/717/727/KGSS(HV) then the BF2 will likely be the better pairing for those. I haven't really referenced it much, because it's in a different setup and only works with my normal bias Lambdas, but the Jolida Glass has a similar signature to the BF2 but is really quite amazing at three-dimensionality - but that is a warm DAC with a solid state speaker amp, so I find it more balanced and plays with the high end of the NB Lambdas better.
One example: I was A/Bing between the D90 and BF2 yesterday, listening to Soolaimon by Neil Diamond, a song I've listened to many times. The drums at the beginning had a more natural sound on the BF2, but on the D90 for the first time I felt like I could clearly hear that they were being struck twice (ie, the four fingers and the thumb, separately). A microsecond or 10 apart. I went back to the BF2 and now that I knew what I was listening for, I could detect it there too, but they were much more blended together and the separation between thumb and fingers wasn't as apparent. It felt more like the hand hitting it once than two parts striking slightly separately.
Re: the sorbothane, I cut them up into little 1-2cm squares, and would place 2 at a time all over the plastic housing (all on the outside). Some where the cable enters, some on the headband, some on the thicker plastic strips running along the outside rim of the housing. At first, it was a disaster (like when you bang a bongo drum that is sitting solidly on the floor, no reverb or whatever that is). As I removed some, it got much much better and now I just a have a few of them scattered around. It tames the reverberation (and I think distortion) but too much is worse than too little (thanks to John Wimberley for the guidance, though they talk a lot about that in the original thread I linked to).