STAX Lambda Pro or Signature Earspeakers

Hi Fellow Audiogoners - which classic STAX Lambda earspeaker is best - the Pro or the Signature? Which energiser maximizes their performance? I look forward to your timely reply.
I have owned both the Lambda Pros and the Lambda Signatures. IMO, they sound very similar. I sold the Pros with the SRM1 Mk2 driver unit because I much preferred the Signatures with a tubed Stax unit (SRM T1S). For my money, the tubed driver unit was the difference not the phones.
I have owned both for many years and drive them with the SRM T1S dedicated tube amp. Both sound fantastic and while it is not a night and day difference, the Signatures are clearly superior sounding with a noticeably more refined sound, more speed, and superior bass definition. They were also considerably more expensive than the Pros.
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