I own a Sander Magtech amp but have not used it on electrostats.
That is good because the Magtech is actually designed for dynamic speakers not ESL's. Mr. Robert E. Greene of TAS used the Magtech during his review of the Kingsound Prince II speakers which is really a bit unfortunate when Sanders actually builds an amplifier dedicated for ESL speakers call the "ESL Amplifier".
I have listened to both the Magtech and ESL amps and both offer outstanding value. Back to the OP's question i would take the Sanders over the Bryston