My experience adding subwoofers to 2 channel

I wouldn’t even consider a tube amp to power subs due to their generally low damping factors and their resulting poor ability to firmly control the movement of dynamic cone drivers, especially the precise starting and stopping required of larger and heavier bass drivers in subs.

Actually this 'precise starting and stopping required of larger and heavier bass drivers' is a bit of a myth. You can get excellent and natural bass out of a tube amplifier if you set things up right. Our amps go full power to 2Hz so no measurable square wave tilt at 20Hz. So often what causes people to think tube amps don't make good bass is phase shift; if there is a cutoff within 1/10th the lowest frequency to be amplified phase shift will be present. This can cause a lack of impact. FWIW, if you overdamp the speaker it will be less able to play bass impact correctly; no speaker needs more than 20:1 damping factor.@mijostyn  Some of those Acoustats were very easy to drive and some weren't. I've never figured out the models, but a customer had a set of M-60s driving Acoustats and the combo was wonderful. The Acoustat was a very nice and IMO an undersung product.

Hello atmasphere,

Thanks for the good info, I learned some good stuff.

Tim

Atmasphere, they were an undersung product but the company had trouble finding itself and was trying to be all things to all people.  Initially all the Acoustats used a "direct drive" high voltage tube amplifier. The first Model was the "X"  It was a beautiful 3 panel speaker with the amp mounted inside a plywood enclosure. Unfortunately it was very colored entirely due to the enclosure. They then moved to the Monitor series both three and four panel versions. It was the Monitor 4 which convinced me that they were on to something. It was with this speaker that I first played around with subwoofers. They then again redesigned the enclosures  to make them easier to manufacture. These were called the 1, 2, the 3, the 4 then the 1+1, 2+2, 3+3 and 4+4. They felt their sales were low because people wanted to use their own amps so they came up with a two transformer interface which was relatively easy to drive. There were two versions of this interface. The problem with this was that us audio types preferred the Direct Drive amp. The smaller speakers unaided by a subwoofer were not impressive. But the 2+2 was almost an entirely different animal. Given an amp like the Krell KMA 100 they not only had the magic ESL midrange but killer dynamics. Put them with a subwoofer and all hell broke loose. As big as they are they remain very selfish speakers. Very few 3+3s and 4+4s were made. They were just two big and you had to run two interfaces and two amps with each speaker. The 4+4s were the size of the Soundlabs Majestic 845. The most important item that Acoustat brought to the world was the first totally indestructible electrostatic panel. Quads had given ESLs a bad name because they were very fragile and I think Acoustat suffered because of that. My panels are now 40 years old. The 1/2 life of the plastic used in their construction is 50,000 years. Unless you drive a stake through them they will go on forever. If you have heard any of the larger SoundLabs then you have essentially heard the 2+2 but with a bit more dispersion. Most of these speakers have been cannibalized and the panels beaten up. 2+2s in serviceable condition are hard to find. The Company was eventually sold to David Hafler who went belly up some time later.
Panel speakers have gained more acceptance and I think there is room for a lower cost full range ESL like the 2+2. The individual panels might cost $50 to make if that. The frame maybe $100. The transformer/power supply another $100. $400 all told for a speaker you could sell all day long for $4000.............
 

Music Reference now offers an ESL with the option of a direct-drive (no ESL transformer) OTL power amp. MR's Roger Modjeski designed the same for the original Beveridge, and has custom made some d-d amps for Acoustats.

@bdp24, the Beveridge amps were not Roger's design, only the RM-1, RM-2, and RM-3 which mad up the rest of the components in the Beveridge system which if I recall was the 2SW.