When it's there your ears know it, but words don't seem to be able to describe it-- its the soul of music. We've have playing a build up of the Simple 300B as described in this post.
Some more on the circuit:
This is not "typical 300B" operation and needs a 5K load. Power is around 7w at <3% THD. 
The V/I ratio is more like 45 than 300B (250v/40ma scaled up to 350v/60a) and, dare I say it, sounds more like a 45 but with much more heft.
Actually it is a WE Data suggested operating point. WE notes 7w @ 4K load with around 3.3% 2nd HD. With a 3K load you get 8w at 5% THD.
Crank the current up to 80ma into 2.5K and you'll get more traditional 300b sound and 10w Class A1 or more if you drive the grid positive.

So we've got the FCUPS providing the pulse, DIYHFS filament supplies on the 300B, CCS modules on the paralleled 6SL7 halves and ABS module supplying fixed bias.
I know, all that solid state sand gives us traditionalists the shivers, but anyone local is welcome to come in and have a listen.
My ears tell me there is very little distortion; it hangs together famouslly, never sounding congested. Bass is taut and goes waaaay down low-- can this be 300b?

I still need to tweek the layout a bit. All the AC power stuff is on the right hand side of the chassis and the signal output stuff is on the left. We may be able to get a bit more p-p swing from the 6SL7 CCS by tweeking current too.

Well I like it. very much. Vocals never sound strained, strings flow with silken timbres, bass transients startle. And all this is on a little pair of 5" Feastrex?

OK I've just added the UTS 6db linstage into the mix. More on that next posting.