Output threshold using a Dynaco ST-70 amplifier with passive pre.

I am concerned with overdriving my recently refurbisher Dynaco St-70 amp. I is my understanding that the GZ34/5AR4 tube based rectifier is the 'weak link' regarding this amplifiers output power capabilities. Am I right on that (Question 1)?

I am using a HRT Music streamer Ii which has a maximum output of 2.25 volts. This is connected via short interconnects (0.5m or less) to a Schiit Audio SYS passive pre which is in turn connected to the amp via short interconnects. Source is Macbook pro via USB.

Question 2) At what input voltage might I be overdriving this amp?  Sensitivity rating states "1.3 volts rms input for 35 watts out".  I am thinking that the 1.3v may be at the upper limit given this amp is rated 35wpc.

Question 3) Can I measure the output voltage on the SYS pre at at various output levels as driven by the DAC using digital meter without harm to pre or DAC?  I am thinking of using RCA connecters on pre outputs having striped wire ends for attachment of meter probes.