Primary impedance is choosen 3...7x Tube Rp. Choice is mandatory, useually taken "as available" from some manufacturer. So when you read "5700 Ohms" somewhere, that doesn't mean 5.5k or 6k sound bad. Even so, you will not notice the difference. 10..20% change doesn't matter much, and either way it is not worce or better. You better should ask for the inductance (Henry) as this determines the low response.
Paralle Single Ended: