The SR-30 was the entry level electrostatic phone from Stax. Believe they are about 12 years old. The SRD-40 & 84's are electret - not electrostatic and do not sound as good. The Lambda's are great phones and IMHO are much better than the models you mentioned. Another good vintage Stax phone is the SRX-MKIII but you don't see many of them used.
