An typical Stax setup is about on par to slightly better a poorly syngergized HD800 setup. Things change drastically once you bump up the notch for the HD800's with complimentary amps and source but so does the price.
Nah, the new series not only has slightly more bass quantity but also better bass quality. In fact I distinctly remember one of our most distinguished members citing that the SR-507 has cleaner bass than the "typical" Lambda. I don't remember any negative impressions of the 507 actually, besides the fact that it wasn't an Omega.
Define better bass quality. The only reason why those same people say the newer 407 or 507 have better bass quality are comparing to LCD's or HD800's most of the time, comparison between it and the older line are only done by a handful bunch of people here such nerdling, Stax mafia and non active low count posters. Buy 404LE/507 (they use the same leather pads) put it any Lambda model and you will instantly have more bass.
As I said before, the old vintage Lambda models are more, balanced, neutral and realistic sounding then the modern Lambda garbage Stax has been producing since the last good Lambda and that was the LNS. The x02 series ain't half bad but Stax went back to the same old with the 207-507 with bumpung up the brightness to give users to perceive more detail retrieval.