A few caveats to my impressions.
-My 300b is an AudioNote Kit-1 and while it has all the upgrades (black gates, great tubes, etc.), I won't pretend it is SOTA.
-I am a lifelong music listener now approaching 60 (groan)! That means I have my own biases and 60 yr. old ears. I actually think my hearing is more acute than ever (hubris?). I definitely focus on the mid-range.
-The F5 is hardwired to a Buffalo/Ivy combination (fed flacs via the SPDIF of a Juli@ at the moment). I have not heard the 300b in this chain. The last time I heard the 300b, it was via an Opus/Aikido combo.
Having said that, I am quite certain that the K1000's have never sounded better. Some of the improvement is due to the Buffalo (really the ESS Sabre chip) but I'm pretty sure its the amp as well.
The improvement in sound is in all the usual criteria. Cleaner, more articulate, delicate, spacious and effortless.
What is most important to me is that it is more engaging emotionally. I am listening now as I type this and I am continually being distracted by the music. While this certainly has happened with my other setups, it has never been to this degree and frequency.
I listen to a fair amount of live (legal) boots. Usually great music with acceptable sound. I am most shocked at the improvement in their sound.
I have listened to my AudioNote for about 15 years, the Tripath 2050 for about 5, a First Watt F4 clone for nearly 2 and the F5 for about 3 months. I stopped doing quick switch comparisons as I have found that listening in a non-pressurized setting over time is what is most important.
I could go on but this is already too long. Just a final thought. I am NOT saying that a well executed 300b in this chain wouldn't sound as good. It probably would. BTW, an F5 clone can be built for about the cost of a good set of 300b tubes thanks to the unbelievable attitude of Nelson Pass to permit/encourage DIY clones of his designs.