12AU7 and related 9 pin triodes have a center tapped filament that can run on 12 or 6 volts depending on whether the segments are wired in series or parallel. In most amps they run on 6 volts as that saves having a special 12 volt secondary winding on the power transformer.
6SN7 draws 600 ma of filament current. A 12AU7 draws 300 ma filament current in parallel mode (6 volt operation).
Simply replacing 12AU7s by 6SN7s would double the filament current drawn (3.6 amps vs 1.8 amps for 6 tubes). That could easily burn out the power transformer if its rating is marginal.
I would follow the manufacturer's recommendation as mentioned above.
6SN7 draws 600 ma of filament current. A 12AU7 draws 300 ma filament current in parallel mode (6 volt operation).
Simply replacing 12AU7s by 6SN7s would double the filament current drawn (3.6 amps vs 1.8 amps for 6 tubes). That could easily burn out the power transformer if its rating is marginal.
I would follow the manufacturer's recommendation as mentioned above.