I've been through this 300B/845 comparison, trade-offs, etc. to the point of having 300B and 845 amps in my house right now. I have a few months of active back-and-forth experience with both tubes now.
My comments here are informed by the experience of using both amps on highly revealing speakers with 101db/w/m efficiency, so I don't need more power than 300B SET.
I have used a variety of 300B amps and generally they share emotionally-communicative midrange and vocal traits, are articulate with delicate detail in the treble and a lush, euphonic "spray" to the top end. Bass is warm but on most 300B designs is falsely rich due to euphonic harmonic distortion below 80 Hz or so. This can work in favor of small speakers with limited bass range, but is decidedly fat on a full-range speaker.
By contrast, every 845 SET amp I have heard has much firmer bass with superior definition over the 300B SET designs, and it's not just an issue of power difference. Woofer control is better. In the midrange, 845s have the usual SET attributes of intimacy and clarity, but it can't quite equal the 300B's uncanny holography in vocal projection. However, transient impact is discernibly meatier than 300B. On top, the 845 is absent the 300B's "spray" so it does not sound lush, instead it has a more objective, drier presentation. Overall, 300B SET amps and even push-pull versions err on the side of romance and bloom in exchange for maximum realism on human voice and remarkable spaciousness in imaging. 845 SET amps err on the side of astringence and less expansive spatial presentation in exchange for delivering dynamic punch, cleaner bass and more precise locations in the sound stage.
There are many exceptions: First, 845 amps are dramatically affected by your choice of 845 tube. Putting aside the scarce and dear NOS RCA 845, the Chinese 845B sounds dark but full-bodied while the newer 845M metal plate is bright and alive, more like a sparkly 300B. The KR 845 is something else entirely, explosive but a little icy, highly articulate and fast. 300B tube variants have their distinct differences but you have much more variety with less drastic differences available for tuning through tube rolling.
Second, some more exotic 300B designs successfully overcome the tube's euphonic inaccuracies to vault into the first rank of amplification. I have Audion Black Shadow 845 monoblocks, which are the best 845 amps I've heard from any vendor. I use the 845M tube in them, which only amplifies their appeal. The Audion Golden Dream 300B monoblocks, however, are overall a compellingly superior amp, one of the two best amps I've ever heard in over 30 years in the hobby. The Golden Dream has a proprietary circuit to sharply reduce the harmonic distortion and resulting bass bloat common in 300B amps, as well as to distill out most of the false lushness in 300B treble. Midrange is as good as it gets. It is a parallel single-ended 300B topology, using 2 300B tubes per monoblock for 25watts. This amp also has no electrolytic capacitors anywhere, with a silver signal path, silver choke and silver secondary winding on the output transformer. It is 1/3 more expensive than the similarly-designed Black Shadow 845 and its bass is at least as tight and defined, with punch and body rivalling the 845, superior image and locational precision, spellbinding midrange and a top end that is equally objective but not a trace dry.
However, a lesser Audion 300B SET, while excellent in its own right, cannot match the accuracy of its 845 big brother.
Audion amps share common characteristics of fast transient behavior, high articulation, tonal richness and dynamic aliveness. Some 300B designs from other manufacturers are similar, others are syrupy and slow. They make a much less expensive parallel single-ended monoblock 300B amp rated at 18w that has the usual lush 300B signature only a little less so. It's very good. SOME people would prefer it over the 845 amp, but not me.
For now, I own both 300B and 845 amps. I listen to 845 much more than 300B, but then I don't own the Golden Dreams. If I did, 845s would see much less action.
I've heard a few inexpensive 845 amps which sounded comparatively indelicate and ham-handed to me, with otherwise nice bass. On balance, at, say, $2500, a 300B SET is likely to be more satisfying. But somewhere between $3000 and $12,000, an 845 can play to its strengths and be the preferred route to fidelity. And then above that price, it becomes feasible for manufacturers to use a variety of techniques for building to the strengths of the 300B while convincingly addressing its liabilities. At least that's what looks to be the case right now. Chinese amp economics can change the order of things very quickly.
Phil
