What makes the 45 or 300B "hard to drive" is that it is good practice to have the majority of amplifier distortion in the final power device, not the driver. In other words, the driver should be cleaner than the 45 or 300B ... which are the lowest distortion tubes ever made.
In a feedback amplifier, the source of the distortion doesn’t matter much ... the feedback sweeps it all away. Which is why the substitution of the higher-distortion 12AU7 for the lower-distortion 6SN7 in the mid-Fifties didn’t matter much, since feedback was in universal use by then, and it didn’t show on the low-resolution distortion analyzers of the day.
To be honest, zero-feedback amp design is kind of a cult audiophile thing. For that matter, any kind of tube amp is a cult audiophile thing. If distortion numbers come first, THX or Class D are the answer, end of story. Don’t mess with tube amps, just buy the solution off the shelf.
It’s an esthetic decision to build zero-feedback amps, whether bipolar transistor, MOSFET, or vacuum tube. I think it is good practice to design low-distortion zero-feedback driver sections, but I have seen (but not heard) all-transistor amps, with lots of feedback, used as 300B driver sections. Which begs the question, why use a 300B at all, if it’s just an expensive distortion generator?
Returnng a little more seriously to the original question, there are a lot of SET amplifiers with marginal driver sections. I’d go out on a limb and say the majority of 300B amps on the market sound mostly like overstressed driver sections, not like a 300B.
That’s why some of this discussion might sound like we are at cross-purposes. If state-of-the-art SINAD numbers are your goal, please look elsewhere. Forget all tube amps, whether pentode, triode, or hybrid amps.
If you want a taste of "tube flavor", get a preamp with a 12AU7 in it. I own a charming little Xduoo TA-10R, which is a AKM 4493 DAC, a 12AU7 gain stage, and a simple two-transistor Class A output stage. 2 watts per channel, sounds great, and all for $320 from Apos Audio. Lots of power to drive planar headphones, and a fun alternative to the usual Topping or SMSL.
Designers of zero-feedback 45 or 300B amps have different goals, which are also different from designing re-creations of Golden Age PP pentode amps. A lot of it comes down to esthetics and design philosophy.