I've been through this kind of transition in the past couple of years, slowly making a transition to high-efficiency speakers + SET amplification which was fully completed in 2005. I began a few years ago by buying KT88 and 300B single-ended amps to experiment with on a second system set up in a small space. A move allowing me to replan my audio systems from scratch instigated completion of the transition with the purchase of Zu Druids for one system and Zu Definitions for the other. Of course, going to 101db/w/m efficiency simply forced attention back on the amplification.
In a 24' x 12' room, 7/7 watts of 300B power went quite far. And the same was true in another room, 20' x 14' I tried a number of 300B amps and some were quite slow and undynamic, while others were quite lively. My experience has been that there is remarkable variety of sonic signatures in 300B amps. The liveliest 300B amp for me was by Audion. Their circuits all have a characteristic fast, dynamic sound.
However, a single 300B per side didn't have as much ability to project sound into the room and push snap energy into the acoustic space as a single-ended KT88 amp. I had a couple of those, one by Acoustic Masterpiece and another by Audion. The Audion amp had 18/18 watts per channel, by comparison. It had firmer bass than the 300B and more dynamic aliveness to the sound, but didn't have the ultimate focus and intimacy of the 300B.
In a conversation with Sean Casey at Zu, on another topic, he casually mentioned that he really liked the drive of the KR T1610 tube, available in a Cary or KR amp, and second to that he liked the 845 tube. I hadn't heard either up to that point in time, and wasn't really thrilled with the idea of having a tube amp with $1800 power tube replacement costs, so ignored the T-1610 for now. I went looking for an 845 amp and coincidental to my interest found that the last pair of factory Audion Black Shadow 845 monoblocks were available. I bought them. At 25 watts of triode power per amp, I then had the holistic sound of an SET with the muscle of a KT88. This was a quite lively and dynamic sound on Zu Druids.
I went through the same experimentation on the Zu Definitions and ended up with 30 watts per side via a pair of Audiopax 88 monoblocks, which use a dual KT88 circuit that ingeniously models triode behavior from a commodity pentode.
I thought I was settled. And then I began to hear some of the very high end 300B amps, which were at prices that allowed the designers to get more out of the tube. I finally got the dynamic behavior I wanted in a 300B, from a pair of Audion Golden Dream Level 6 parallel single-ended monoblocks, each using a pair of 300Bs for 25 watts per amp.
I was fortunate to have the resources to churn through some auditions in my home to settle my amp/speaker match. I can say that even for a 100+db/w/m speaker, having 25-30 watts on hand sounds like a lot more drive than having 6 or 7, even though the actual SPL the power should drive isn't large. You get more of the sense of reserve and effortlessness that you hear putting something like McIntosh MC1201s on an 88db/w/m mainstream speaker.
I observed a few other things. One is that most 300B amps are change their signature quite noticeably from tube to tube. Various KR 300Bs were dynamic and had a hard attack on transient information, whereas a Full Music or Sophia mesh plate tends to be very smooth. I haven't listened to current production WE 300Bs yet. Some of the very affordable Chinese 300Bs, including the Shuguang solid plates have a lot of snap and sound quite lively, while giving up the lushness that many people buy 300Bs for. Point is, if you have a 300B on efficient speakers, tube rolling has to be part of your strategy for getting the right sound for you.
Also, I had a chance to try a few PX25 amps. At 6/6 watts, the PX25 is distinctly livelier than the 300B, with much less bass bloat. In fact, the PX25 amps had some of the best tube amp bass I've heard from any design. That tube had a very clinical, objective sound, but conveyed less spatial information than a 300B. If I hadn't had the means to look for and find more triode power, I might have settled on a PX25 based solution.
If you're on a budget, don't ignore single-ended pentode/tetrode amps. They solve a lot of problems evident in entry-level 300Bs. For instance, at a local Zu demo held in a 24' x 36' x 16' hotel event room, a little $800 Almarro A205 amp was very impressive with all of 4.8/4.8 watts. That amp has just 3 tubes, with power coming from an EL-84 per channel. While a watt is a watt is a watt, the dynamic behavior of similarly-rated single-ended amps varies considerably by tube and design.
The last thing I did was to try 100w - 500w solid state amps to see whether a dramatic power increase would gain anything useful. What I gave up wasn't worth the small amount of reserve that was gained, however. The same was true trying push-pull tube amps. 25 - 30 watts of SET on a 101db/w/m speaker has plenty of dynamic projection in rooms my size.
Phil
In a 24' x 12' room, 7/7 watts of 300B power went quite far. And the same was true in another room, 20' x 14' I tried a number of 300B amps and some were quite slow and undynamic, while others were quite lively. My experience has been that there is remarkable variety of sonic signatures in 300B amps. The liveliest 300B amp for me was by Audion. Their circuits all have a characteristic fast, dynamic sound.
However, a single 300B per side didn't have as much ability to project sound into the room and push snap energy into the acoustic space as a single-ended KT88 amp. I had a couple of those, one by Acoustic Masterpiece and another by Audion. The Audion amp had 18/18 watts per channel, by comparison. It had firmer bass than the 300B and more dynamic aliveness to the sound, but didn't have the ultimate focus and intimacy of the 300B.
In a conversation with Sean Casey at Zu, on another topic, he casually mentioned that he really liked the drive of the KR T1610 tube, available in a Cary or KR amp, and second to that he liked the 845 tube. I hadn't heard either up to that point in time, and wasn't really thrilled with the idea of having a tube amp with $1800 power tube replacement costs, so ignored the T-1610 for now. I went looking for an 845 amp and coincidental to my interest found that the last pair of factory Audion Black Shadow 845 monoblocks were available. I bought them. At 25 watts of triode power per amp, I then had the holistic sound of an SET with the muscle of a KT88. This was a quite lively and dynamic sound on Zu Druids.
I went through the same experimentation on the Zu Definitions and ended up with 30 watts per side via a pair of Audiopax 88 monoblocks, which use a dual KT88 circuit that ingeniously models triode behavior from a commodity pentode.
I thought I was settled. And then I began to hear some of the very high end 300B amps, which were at prices that allowed the designers to get more out of the tube. I finally got the dynamic behavior I wanted in a 300B, from a pair of Audion Golden Dream Level 6 parallel single-ended monoblocks, each using a pair of 300Bs for 25 watts per amp.
I was fortunate to have the resources to churn through some auditions in my home to settle my amp/speaker match. I can say that even for a 100+db/w/m speaker, having 25-30 watts on hand sounds like a lot more drive than having 6 or 7, even though the actual SPL the power should drive isn't large. You get more of the sense of reserve and effortlessness that you hear putting something like McIntosh MC1201s on an 88db/w/m mainstream speaker.
I observed a few other things. One is that most 300B amps are change their signature quite noticeably from tube to tube. Various KR 300Bs were dynamic and had a hard attack on transient information, whereas a Full Music or Sophia mesh plate tends to be very smooth. I haven't listened to current production WE 300Bs yet. Some of the very affordable Chinese 300Bs, including the Shuguang solid plates have a lot of snap and sound quite lively, while giving up the lushness that many people buy 300Bs for. Point is, if you have a 300B on efficient speakers, tube rolling has to be part of your strategy for getting the right sound for you.
Also, I had a chance to try a few PX25 amps. At 6/6 watts, the PX25 is distinctly livelier than the 300B, with much less bass bloat. In fact, the PX25 amps had some of the best tube amp bass I've heard from any design. That tube had a very clinical, objective sound, but conveyed less spatial information than a 300B. If I hadn't had the means to look for and find more triode power, I might have settled on a PX25 based solution.
If you're on a budget, don't ignore single-ended pentode/tetrode amps. They solve a lot of problems evident in entry-level 300Bs. For instance, at a local Zu demo held in a 24' x 36' x 16' hotel event room, a little $800 Almarro A205 amp was very impressive with all of 4.8/4.8 watts. That amp has just 3 tubes, with power coming from an EL-84 per channel. While a watt is a watt is a watt, the dynamic behavior of similarly-rated single-ended amps varies considerably by tube and design.
The last thing I did was to try 100w - 500w solid state amps to see whether a dramatic power increase would gain anything useful. What I gave up wasn't worth the small amount of reserve that was gained, however. The same was true trying push-pull tube amps. 25 - 30 watts of SET on a 101db/w/m speaker has plenty of dynamic projection in rooms my size.
Phil