A few misconceptions.
A Lambda headphone will sound damn good from a relatively "cheap" amp. If you have a speaker amp, you could get a used Lambda and SRD6 transformer combo for $225-300. The O2 benefits the most from a great amp and is by far the most power hungry headphone I've owned or had extensive time with. Sigmas are also power hungry.
I wouldn't trust reviews much at this point just because the LCD2 is still the FOTM. I don't remember the last time a new headphone wasn't called "the best", a "giant killer", or "better than <some highly regarded headphone>". Lambdas are over 30 years old and still popular. If people are still hunting down LCD2s in 30 years I'll be very impressed.
As for the description of the Lambdas, I can't say I've heard a Lambda that sounded like that. I agree they are bright, but so is almost every headphone to my ears. The bass on an electrostat is one of the best things about them to me...I think people who say Lambdas have no bass either haven't heard them or are used to the God-awful monotone bass bloat exhibited by cans such as Denons. If they had wonky mids, no bass, and were bright then who would like them?
I'm not sure I've even read a negative overall impression on a Stax headphone before, which says more than anything.
I do agree that the LCD2 is awesome for the price though.
As for the SR507, I have high hopes for this headphone. I didn't buy into the 404LE hype train, but this headphone looks like it has significant (or at least more substantial) upgrades.