To answer the question about which Lambdas, I have had tried all the Pro bias vintage, and a few of the new ones ...of the vintage, the LNS is the best for me; really sings with the 007A amp or T1. (best midrange bloom, least harsh highs, bass is there, but not in spades). I also, on occasion, like the Gamma Pros; not a musical as the LNS; with a much different sound signature from the Lambda's, and not quite as open...it's just that sometimes I like the change.
I much prefer the LNS to the 404's or 404LE (never tried 507). I now have Omega's II's on the way...will do a long A/B vs the LNS and sell the one that finishes 2nd (who am I kidding, I'll probably keep them both).
That said, paying attention upstream to your source and DAC will ultimately be the determining factor. On the Chord DAC, they all sound very good; as the Chord is great at getting rid of sibilance in poorer CD's. With the fully loaded Empirical Audio Overdrive fed by a good transport, the LNS is able to bring out the detail better than other Lambdas (thinnest membrane in the LNS compared to other Stax).
I use rather expensive PS Audio power conditioning and good cables (Stealth, Kimber Silver Streak), which are necessary with a revealing DAC, as the Empirical (or other revealing DAC) will sound shrill with all but the very equipment in front of it.
So...I recommend buying/trying all the Lambdas and selling what you don't like. There's a good market for used and I've almost always recovered my money by buying used and then selling to upgrade/try something else.
Too many times on here I read that one amp/Stax phone is better than the other...without proper care/thought given to the rest of the audio chain, and especially the front end.
I also run a subwoofer (Rel Stadium) with my phones, as none of the family is too near my dedicated listening room (or man-cave as my wife calls it). LNS + 007A + Rel sub to me sounds better than anything else I've heard at ANY show/meet (including the BHSE + 009 without a sub). Of course, I'm sure the 009's with a sub would be heaven. I know the sub idea has been kicked around before and largely dismissed by some as weird, dumb or not practical...but once you hear a properly "loaded" room with headphones this way, you are hooked. BTW the sub does not "boom"...it merely fills in the bottom that's missing and adds needed punch to the music that no small (ear) speaker is capable of. If you look at all of the prominent electrostatic speakers (Soundlab, Quad, ML, etc.) most of them need a very fast sub to fill out the bottom.