Primary impedance is choosen 3...7x Tube Rp. Choice is mandatory, useually taken "as available" from some manufacturer. So when you read "2500 Ohms" somewhere, that doesn't mean 2k2 or 3k sounds 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.
Parallel Single Ended:
How to replace a 2A3 by two 45's. Nice design idea! Read more here