Why I used 2x717A per channel while only one is good enough ?

717A, 310A, EF86, D3a, C3g as well as all other Penthode tubes has advantage of high gain, but the biggest disadvantage of penthode is tube inside impedance is too high, if using it as driver tube, the response frequency will be dropped down very fast from 10K Hz. In almost case even that it can be matched with almost speaker without any problem because almost speaker and human ear, the sound is good if the response frequency from 50Hz to 10KHz is good. That is why you can see many amp use only one penthode as driver tube.

But as you require a wider range of sound, you need a solution to this problem. The easiest method is using NFB to make a better output data. Because the gain of penthode tube is very high so using NFB does not affect much to the total gain of amp. But it is not interesting method actually.

Using triode connection if you dont want much gain to driver power tube. BUt this is not good too because to do so, the gain of penthode tube will be same as triode tube, not good at all. You can use another technical method, as cathode follower, using one triode tube as buffer tube to connect with 300B, a triode tube with very high input impedance so the signal from penthode tube go to triode tube will transform from very high impedance to lower impedance to driver output power and the drop down in high frequency will be resolved. But using cathode follower make the schematic more complicated and difficult to use.

In this amp, I dont use cathode follower method, the triode tube was used is penthode 717A which is triode connection so the sound of 717A will not changed as a driver tube. Triode 717A in this amplifier is acting as loading resistance for penthode 717A, the schematic is almost same as using one 717A tube, as simple as possible.