In that schematic, the 33K Ohm resistor is a grid stopper and not connected to ground. A dot is a connection point. It is connected directly to the pentode grid there to keep the input tube from oscillating.
The input impedance in this case it 1 Meg Ohm. The capacitor would be 3.32nF (3300pf) for a high pass F-3dB of 48Hz
Yep, but you may not want to set the -3db point this high. This will drop your bass even further (you'll be down 6db at 48Hz instead of 3db) from your ADS speakers, unless you want to rely on your subs for bass in this range. This will also cause fairly significant phase shift at and above this frequency. A first order filter will result in a 90 degree phase shift, centered at the cutoff frequency. In other words, at 48Hz, there will be a 45 degree phase shift. You'll still have a 30 degree phase shift at double this frequency.
I'd suggest picking a somewhat lower frequency (maybe 30-35Hz). This will still reduce the amount of wasted power below where your speakers can generate meaningful output, but won't limit their extension nearly as much and will also reduce the phase shift in the audible range.
Phase shift at these low frequencies probably isn't going to be very noticeable (if at all), but unless you are pushing your amp pretty hard, I'm not sure the small power benefits of a higher cutoff frequency justify the reduced low frequency response from your mains.
Since your input impedance is quite high and the cap value you need is quite small (and therefore less expensive), you could get a few different values and try them out to see what you like best. The bigger the value, the lower the cutoff frequency, so you'd want to try values between 3300pf and 6600 pf.
My two cents.
