300B or 845?

am new to tubes.
I have read the archives.
Can anyone explain the inherent differences between the sonic character of 300B and 845 tubes?
Given the amp cicuits are good and built to utilise the strengths of the tube - what differences are audible on 96db sensitivity speakers?
My music is all acoustic - and 90% is female vocal. I love the sound of a sweet intimate vocal - there are not that many - plus single instruments: guitar, viola, flute, fiddle. I like melody and rhythm. I don't cope with massed instruments be it orchestral or rock bands.
Any advice is deeply appreciated.
Spencer, if you chime in, I'd like to know what you think of the SET performance versus your Atmaspheres.
Been waiting over 10 years!
As an importer and re-seller for a manufacturer of 845 and 300B amps they are both amazing tubes.  One of the most important questions in terms of soundprofile and overall amp quality is going to be in regard to the transformers.  They are key to an amazing SET

The 300B is going to be your most linear and musical tube but is going to be limited in the amount of power it can deliver.  The soundstage is gorgeous and lush and the midrange/treble shines like the sun.  Bass can be a hair flabby, especially compared to an 845. A good 300B SET is going to be ~10w.  With a 96dB SPL Speaker though, that is a max output of over 102dB. Plenty for any normal person. 

An 845 SET is going to give you tighter bass but is going to be less lush.  Compared to a SS or even a 6550 or KT-88 based amp you are going to get much better PRAT and spectacular midrange/treble but not quite what a 300B delivers.  You get a bit more power out of an 845 and they do tend to be slightly less expensive amplifiers.

The Art Audio Diavolo is 10w and my Carissa is 18w.  

Tube costs vary wildly for both tubes.  A good pair of 845s range from <$200 a pair to $1900 and 300Bs can be had for $200 a pair to $1600.  Quality of the tubes will make a difference and it matters 

A 2A3 or a PX25 (similar tubes) are arguably as linear as a 300B with tighter bass and the same lush soundstage.  They tend to be lower in power output and usually are 5w or less.  This would still have you 99dB of output at full power 10' from the speaker.  

I am re-introducing a PX25.  The advantage again is that it is a bit cheaper than a 300B though you have a lot less tube choice but the two that are available new (Sophia and KR) are spectacular.  

One other note - What speaker are you running?  There are a couple high efficiency speaker manufacturers who are notorious for over-rating the SPL on their speakers.   I would hate for you to get a 10w amp when you really need something more powerful because the speaker is really 91dB instead of 96dB.