Yeah, if you're going down the interstage route, then the soft start driver board doesn't make a whole ton of sense. You'll end up needing a ton of current and a reasonable amount of voltage, which would produce too much power across the C4S.
The Tango NC-20 is a 1:1 interstage. The 45 has a mu of about 3 nominally, so you need a ton of step-up between the driver and the output if you want full power, or you need a third tube.
Then, you'll have to consider that interstage transformers typically don't go much above 30mA. Since you want voltage step-up, you'll need to drive a low impedance with the 45 to accomplish that. When you look at the 45 curves, then start drawing these steep load lines, things get pretty ugly in terms of distortion.
The one other option I can think of is to add two transformers to the circuit. One would be something like the Sowter 8423 to sit between the 45 and 300B. This IT will handle a lot of current, so you can get out of the nasty areas on the curves, but it won't give enough step-up. To get the step-up, you'd also want an input transformer. Without siting down to crunch the numbers, a 15k:600 wired with the 600 at the input will get you some additional voltage. All of this working together will get you back in the ballpark of reasonable gain, but at the cost of:
Interstage iron: $420
Input transformers: $150-300
New chassis, layout, prototype time for quietest transformer orientation:
If you want all directly heated Paramounts, it's doable with 3S4's as pentodes or other various directly heated pentodes run as pentodes.
