The issue with the S.E.X or Declare is the low voltage output. The SRD-7 has a 25:1 transformer step-up, which means that 1 volt in results in 25 volts out. The Bottlehead is spec at 2 watts output, with a 16 ohm transformer output that means about 5.6 V RMS. The Decware is specified at 2.3 watts output into 8 ohms, which corresponds to a bit more than 4 V RMS. This puts their max output through the SRD-7 at about 100-140V RMS.
Note that the smallest Stax amp, the SRM-252 is specified at 280 VRMS. To match that, you need an amplifier that puts out at least 16 watts into 8 ohms into the SRD-7. The bigger Stax amps are specified at around 340-350 VRMS, which corresponds to around 25 watts into 8 ohms. The original Stax SRM-T2, which has the most voltage output of any Stax amp built, is specified at 635 VRMS which corresponds to about 80 watts into 8 ohms going into an SRD-7.
What all this means is that you don't need a Phase Linear 700 (350 watts/channel) going into an SRD-7, but something bigger than a S.E.X. would be more appropriate, unless you are limiting your listening to solo voice, chamber music, or soft rock at moderate levels.