Hi,
I've just joined the forum, because I was searching for information on Sound Sales and found this thread.
My interest in the company is largely as a commercial manufacturer of the 1934 Wireless World Quality Amplifier.
http://mike.wepoco.com/retro-geekery...ity-amplifiers This amplifier was redesigned many times until eventually replaced in 1947 by the famous Williamson design. For the Quality Amplifier PX4 or PX25 were the output valves of choice even when 6.3V octals were used for the rest of the amplifier.
In advert in March 1946 Wireless World the "DX Plus 7" Quality chassis is launched featuring a "paraphased PX4 push-pull output stage". It seems you have an amplifier of this design.
Hope this is of some use.
Michael