300b, x45, 211, etc.

Your question is perplexing: what exactly do you want to know?? The product differences? There are very many products using the tubes you mention... The difference between the tubes?? If so, it's difficult to say much more than that the 211 is more powerful & slightly more extended in its linear operating region than a 300b. The 845 is also a more powerful amplification tube... They use different voltages and output trannies. Ultimately however, the sound you get depends on the circuit as a whole -- not only the tube.
You're listening to a 300b already...

Well, I can give you a reductionist, and highly subjective rigmarole based on limited experience with products sporting 300b, 211, 845, 2a3.

All tubes can be usefully operated in a no-feedback circuit. Good stability. So, basically they're VERY good amplification devices (i.e. better on paper than the best transistors for example) despite the need for output trannies. Having said this, more or less, objective piece -- here goes:

300B, good because of availability, reasonably priced tube and trannies, the worst of the lot in extension and sonics. Excellent tube for marketers, but other tubes are catching up. Reasonable linearity and some dynamics. Low power -- but still workable, somewhat (say a 112 db spl 1W/1m speaker). Basically this is a "nearly" tube: it's nearly good, nearly extended, nearly excellent sounding, nearly...
As such it lends itself to many applications -- for this very reason. I hate this tube in any application EXCEPT for powering an upper midrange driver alone. Maybe a tweet, but wouldn't know where to buy suitable output trannies.

2a3: as above for availability etc; good pricing to boot. Much better sonic attributes than the 300b even, BUT minimal power capabilities. Probably has excellent dynamics, bass trable, mid, lush and clear, veil-lifting and all that -- but given most speakrs' abysmal efficiency, you'd need an army of 2a3 to hear them. In which case of course you'd lose some of its quality attributes. Life's tough.

845: not too good availability (but improving), expensive, and variable quality trannies all expensive. Good power and extended linear region. So, so, low freq. (so what -use a mosfet instead). I like this tube -- but it's depressingly expensive in most implementations.

211: now we're getting serious. Availability starting to get better and you no longer have to be Mr Gates or similar to obtain it (but being rich remains an important consideration). Trannies still awfully expensive. High voltages. Now this is where SET leaves the whispers and enters the premier league. Good sonics, good extension and no bulls...t driving capabilities. In fact, it also has reasonably low output impedance to boot!

1610: one you didn't mention. Very serious driving capabilities, better stability with loads than 211 and capable of excellent energy swings within linear operation. BUT, very limited availability.

_______Voila___
Let me repeat that I'm personally comparing these tubes to one another -- so when I say I hate X tube it's an exaggeration and said in context -- right?
And if s/one offers me a 300B, I'll jump on it!
Cheers

For SET amps, I think you have
to consider the amp and speaker as one unit.

Very nicely put! Indeed, in the old times, the speaker came with its amp -- together. Remember those old radiogrammes (or whatever they were called) and radios?

The 1610 is nothing like the 300b. It's huge and made by a company called KR. They also have an amp using it (which is where I heard it). In single tube/channel config they manage +20W / channel. I think they must be using transistor drivers -- but am not sure.
Similarly, the 211 requires a different circuit than the 300b. But it's seriously more powerful.

Of course Biomimetic is right about feedback which can and is used to stabilise circuits... however, one can stabilise a circuit without feedback if one is into that sort of thing. One can't with a transistor for example.

Hi Bio: nope, the speakers were a mess

The tubes were driving a tractrix horn loaded lowther ex4. 120 Hz horn. Then I had to use a suitable bass driver capable of playing the game (that was fine, a Supravox 400 did the trick). Then, not surprisingly, I wanted to get rid of/tame that annoying wizzer-related intermodulation the lowther had by extending the upper end. Of course, wishing to play purist, I couldn't consider sticking a filter onto the lowther, now, could I?
So, I was looking around a tweet to mate well with the lowther. Never found one; many available supertweets, for example, Murata, are low sensitivity -- which in turn means another amp, sonically matched to the lowther amp, but producing ~4x the power or more... a big mess:)

I like your point about the exotic nature of SETs -- similarly a prewar Leica.

for some applications it works well (small room, jazz, simple vocals, etc.) It's just that for a low power amp, you end up running at max power alot and possibly clipping which takes you into a higher distortion range on the amp.

COuldn't agree more.
However, my experience leads me to seek at least 20W before clipping for orchestral music; even with ~106db spl on the spkr side, I still needed extra energy when using a 300b pushed to 7W. And note that the amp was crossed at ~120Hz, (directly connected to the drive unit, passive line level xover).

In other words, I found it necessary to have a headroom of ~10+ db over the speaker's reference 1W/1kHz etc available in the amp.

But of course, a highish load impedance and good phase characteristics help the situation.