If the output stage is the heart of a single-ended amplifier, then the driver stage is the brains. The Monaco 845 has a directly heated driver stage using the venerable type 46 tube. In my experience, listeners generally prefer the sound of the 46 over both the 45 and the 2A3. Comparatively, the 45 sounds too lean and clean, almost sterile, whereas the 2A3 sounds too juicy, loose and fat on the bottom end. The 46 has just the right sonic balance and is cost effective as well. Although the 46 is not current in production and is only available through NOS, Ultra Fi has a sufficient supply on hand for all of your future needs.
In my opinion, the sound of most amplifiers depends more on the sound of the driver stage than anything else, save perhaps the power supply. I know of no other 845 amplifier in current production that uses a directly heated driver stage. Instead, they use octal-based tubes as drivers or triode-wired pentodes, or worse yet, nine-pin miniatures. Why? After all, we all know about the magic of direct heating.
The answer is they don't want to deal with the added complexity of a directly heated filament supply in a driver stage. It's much easier to simply use an indirectly heated, six-volt filament tube and share the winding with some of the other tubes in the amplifier. But, that's about being easy and convenient (not to mention cheaper). It has little to do with what's necessary to achieve the best music reproduction—which is why the Monaco is built with a directly heated driver stage.
