Table: portable_power_unit
Columns: driver_unit_model_no
release_year
output_voltage_rms
inputs
outputs
notes

Question: What year were outputs is 2x pro bias, rca loop out and notes is vacuum tube released?
SQL: SELECT "release_year" FROM portable_power_unit WHERE "outputs" = '2x Pro Bias, RCA Loop Out' AND "notes" = 'Vacuum tube'

Question: How many outputs are there for solid state, battery operated for portable use listed in notes?
SQL: SELECT COUNT("outputs") FROM portable_power_unit WHERE "notes" = 'Solid state, battery operated for portable use'