It's a matter of striking a price/performance ratio that makes sense. If your preamp cost twice what it did, you might not have bought it. Volume of sales is very important. And these changes might improve the sound a bit. But I doubt they'll take it to a level that would surpass the initial ramp-up in sound quality when comparing your preamp to a cheaper, less well designed one.
The coupling caps, resistors, and tubes would be the only changes I would bother with.
The coupling caps, resistors, and tubes would be the only changes I would bother with.