The 845 can go into A2 if pushed with a strong driver, but most amps don't do that (incl. the bel canto I'd say), thus the so-called 40 watts is really more like 20-25. But that's fine, it's more than plenty. IMNSHO 845 sounds better than 300B by a pretty large margin. The tube is just one part of the sound anyway.
SuperTNT doesnt' have distributors over here yet. THey are probably sold out of the 845M fairly soon, they only had about 50 prs to sell in the first batch.
845B and M are nice, but NOS are still better (no surprise), so the reason to have NOS 845s has not disappeared. I know, I have plenty , and wrote reviews on both the 845M and 845B, posted here and on AA. NOS are expensive, $1000/pr is lower than what most will cost - usually $1300- $2000/pr is more like an accurate price of really great NOS guaranteeed 845s. Are NOS worth it? well, depends on your point of view. ;-)
BTW, A2 operation involves positive grid current, and driving a tube past the zero crossing from negative bias to positive bias. Doing this requires a very stout driver stage, and usually an interstage (often, step-down) transformer, sometimes called an IITC. However, any tube (and amp) that crosses over the zero point from A1 to A2 goes through a hysteresis (sp?) area of non-linear operation, aka Crossover Distortion. The best bet is to have an amp that runs in A2 all the time, if one is going to go the A2 route. Among commercial amps, not many do this. Wavac is the most common brand to run A2, but they cross over the zero point, and thus there is a compromise involved. Most SET amps run only or mainly in A1. The max power from an 845 in A1 is about 24-26watts, and that's flogging it at max ratings. Many tubes don't like to be caned that hard, so often the op point is a good bit lower for a margin of safety and tube life.
There's been a fair bit of discussion on 845s in the archives, search around a bit. IMNSHO it's the best DHT output tube available in current production.
-Ed
