Can't really compare them to other stax but i'm really not surprised
i'm still blown away with my stax lambda signature (SR-407) even after listening 5-7 hours a day.
I did tried many planars though from audeze and hifiman and I ended up keeping the hifiman HE6 V2 which I preferred over the HE1000 stealth, HE1000 V2 and Susvara.
Susvara was better yes a bit more refined and had this natural/soft way of presenting things.
HE6 V2 sounded very close to Susvara and I really liked the bass impact of the HE6, for only fraction of the price.
I will still keep my hifiman HE6 (I think) because it can do a full octave lower in the lowest bass notes.
Other than that the lambda signature (SR-407) outperforms the HE6 V2 in every possible way, it makes the HE6 almost sounds veiled and unclear.
I think you are conflating some names.
Older generation Lambdas:
SR-Lambda
SR-Lambda Professional
SR-Lambda Signature
Lambda Nova Basic
Lambda Nova Classic
Lamba Nova Signature
SR-202 Basic
SR-303 Classic
SR-404 Signature
SR-207 Basic
SR-307 Classic
SR-407 Signature
SR-507
The Stax Lambda Signature is a completely different headphone than the 407 Signature.