Which Lundahl "45" transformer to take?

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.

Single Ended:
LL1620-60mA. 60Henry, Primary 6k. Secondary 4, 8, 16 Ohms. 13Hz@1.5dB
LL1663-39mA. 45 Henry. Primary 5k. Secondary 8 Ohms. 16Hz @1.5dB

Paralle Single Ended:
LL1664-80mA. 29H Henry. Primary 3k. Secondary 8 Ohms. 12Hz @1.5dB

Click here for schematics