If using lower voltage, the tube may not operate at the optima performance, the sound may be very slow and strange. Do not suggest to run at 5V. Normally, we can run the filament at lower voltage say 6.1V but never run at 5V.
For A C or DC , it is really depend on what Signal Noise ratio you want.. For pre-amp, I always run in DC mode as it is more stable and less noise, For power-amp, i will run in AC.
Rectifier tube must be AC. YOu cannot use DC for rectifier tube.For signal tube especially on the pre-amp, I normally running DC because it is quite easy to get hum if using AC. Besides, you do not need need to match the AC voltage of transformer to filament if using DC.
For some rectifier tube like 5U4, the positive will come out of Leg 8, which is also the filamenet , it make things very complicated if you are running DC, For some tubes like 6X4, it is fine to run it in DC as the output is separated. But why border to run DC is AC is ok. Basically, I have never seen rectifier tube running DC filament.
Yes, Place of transfomer is good to have one vertical and one horizontal next to each other as this will cancel the EMI of the transformer.
No, Bleeder resistor will normally connect to the +Ve and -Ve of the capacitor. So it mean you need two bleeder resistor if you are using CLC, each one should be around 150K to 270K , at least 2W, better 5W depending on the voltage current. The smaller the bleeder resistor, the quicker the discharge time but the resistor will get hotter, so it is adverisable to use large "watt" for smaller value.
