I think it's because of the seal , 404LE has leather pads, 007 too. Not sure about the Sigma but it has a big, dampened chamber so that might help a lot to build up bass.
Also, I tried the ur-Lambda, the first version with normal bias and dampened earcups and I listened in utter disbelief.
I've listened to Erica de Casier's - Sensational album and after the first few seconds I was like - no, cannot be, this is way too good.
Absolutely incredible tonal balance considering it's age, okay the highs are a bit hot and brash but I've only tried it with an SRM-313 and Tempotec Sonata HD USB dongle playing Qobuz through Roon but still, nothing high-end.
Most incredible was the bass, yes the bass weight - it was good, as it should be. More than in my 404's, even after reglueing the panels. The Lambda NB seems to create more soundstage depth rather than width but that's probably because of the dampening and of course the midrange is a bit cloudy when compared to newer models but still - WOW.
I 've also got an SR-303 to go with the 313 but really I don't know what the problem is with Stax, all glue is completely gone, panels laying loose in the earcups. Also the cable of the 303 is much smaller than on 404's and a nightmare to work with, it's coiling and tangling all the time. I will try to iron it through a thick piece of cloth and low temperature to see if it helps.
The Lambda NB doesn't have the glue issue at all but I had to cut the units off anyways as someone made a nasty dent in one of the protective grilles. Also noticed the panel had a small dark spot on the outside so I need to keep an eye on this if it's not arcing further.
I'm starting to think I'd rather have the Lambda NR than 404's
or maybe try 404's with dampening...
Also played a bit with the 001's but they are at the edge of being a total writeoff, the amp works but the cable is gone (broken in 2 places) and the frame of the 001's is gone, glued and snapped off again, very weak design.