Stax Driver Comparison

Has anyone compared SNR of the Stax SRM-006tII and SRM-007tII to older Stax tube drivers such as the Stax SRM-006t? I want to evaluate 24 bit digital systems using Stax earspeakers and so starting with the quietest Stax drivers will be a good start.

I have an older solid state Stax driver and am interested in upgrading to either the SRM-006tII or (if there is a huge difference in sound) the SRM-007tII. I haven't heard either one. I have SR Lambda Pro earspeakers and wouldn't mind getting a newer earspeaker model either, but first I'd like to know that I have a new quiet driver.

Stax says the new tube drivers use DC heater supplies rather than the older AC heater supplies, reducing power supply hum and thereby improving SNR. They made further other changes (such as extending frequency response) to better match the capabilities of SACD and 24 bit DVD Audio. Yet I haven't found any information on their website or in a review about what the SNR was in older units, and what it is in newer units, or at least how much of an improvement in SNR there actually is in their newer drivers.

Part of the reason might be that they hand-pick front-end FETs for low noise, so what they select could vary from one batch to another and they can't commit to an absolute SNR limit. That's just conjecture though.

By being better able to match the capabilities of SACD (effectively 20 bits per sample) and 24 bit DVD Audio, does that mean their older drivers did not have, say, approximately 120 dB SNR to at least match 20 bit digital samples but that their newer drivers do? Is the SRM-007tII significantly quieter than the SRM-006tII?

Does Stax have a solid state driver that is quieter than the SRM-007tII?

Thanks for any comments.