What improvements would I get with a "better" preamp?

@westcoastaudiophile So............I can't just listen to see how they sound to me? 

@billpete you can do whatever you want / pleased to.. being analog design expert, I am trying to warn, as tube rolling SQ effects could be interpreted wrongly. 

below is an example of load-line of tube in circuit. small changes in tube behavior btw. different manufacturers will cause significant performance impact of the amp. 

@westcoastaudiophile OK, thanks for that. I thought I was going to have to stop listening to music there for a minute. I'm nor sure of the overall harm in my mistinterpretation of the sound quality of a given tube in my collection. I call myself an "avid listener", which I have been for over 70 years. I never intended to build my own gear or even to know exactly how it works. I do enjoy listening. Just so you know, you're not much fun. :)) Have a good one.

no problem bill, just look at the example of design issues of tube amp above (just added)!

@westcoastaudiophile When I look at that chart, I might as well be blind. I still have my ears though and I will continue to evaluate with them. I like to read about tests and I enjoy reading specs but my ears are always my final evaluator. I think yours are too, you just get hung up on measurements. You have the advantage or maybe the disadvantage of knowing the gear inside and out. I only have ears and a very basic understanding of stereo equipment and how it works. I'm OK with that or I'd have learned more, long ago. Sometimes, I do feel like I would enjoy a better understanding of the equipment and how it works but I don't have the patience or maybe even the ability to do so anymore. It's all good. Enjoy. I know I will.