A good place to get started is with the Threshold Cascode 1 at 75 RMS per side or the Cas 2 at 100 per side. For Class A the 400a is very good.These are usually reasonable starting at $400 for the Cas units and roughly $600.00 for the 400a. From there you can go to the model 4000,now it starts to get expensive. There is a web site devoted to vintage Threshold equipment. Well worth reading and the gentleman that works on this equipment is Jon_soderberg@rsvlonline.net. Also his web site is Vintage Amp Repair. Understand he worked at one time at Threshold. His knowledge and undertanding of Threshold is second to none ---Look him up.