For your musical tastes a well put together SET system would be ideal, IMO. Any opinions about the two tubes you ask about will be generalizations and will largely depend upon just how those tubes are implemented within a specific amp design. Power supplies are also very important in a SET amplifier, as in any amplifier. There are great 300B amps, as there are mediocre 300B amps. Same goes for any tube. Assuming you are considering a SET implementation of the tubes you mention, a 96db speaker would be just about right for a smallish to moderate room to enjoy the splendors of 8 watts of SET amplification done right. Synergy of amp and speaker is paramount with low-powered SET amps though so care should be taken - research your selections. I have no experience with the 845 to speak of. I believe the designs that use them are generally more expensive. The tubes themselves are less expensive than 300B's but do not last as long and there seems to be a more limited selection. Check with member Gunbei (Dean) - He's been loving his Bel Canto SE40 which uses the 845 tube. I can tell you my experience with the 300B tube in my system excells in every area you mention. It is not as good in orchestral nor Rock. But with vocals and acoustic solo instruments I have heard nothing better. You can read plenty about these tubes, and different amps that use them, both here in the archives as well as over in Audio(SET)Asylum. I won't wax rhetorical on stuff already said much better than I could say it, suffice to say, good choice...you've obviously done your research.
Marco
Marco