The M-22 Amplifier was a much higher power version of the M-22 with additional refinements and even better specifications.  Nearly identical in appearance to the M-22, the M-25 could be distinguished by the gold plated terminals and the Class AB identification plate at the front of the amp.  A great deal of research and development went into the M-25, and what Pioneer had to say about it in the Series 20 brochure is interesting.  To quote, in part:

"It takes a power amplifier capable of reproducing an astoundingly wide range of frequencies, with especially clean and undistorted response in the highest ultrasonics, to satisfy the tough demands of today's audiophiles.  And unless it can do it from the least to the last of its rated power output, and do it hours and hours on end without showing signs of fatigue, it can never merit the term "excellent," not by Series Twenty standards.

"The "Magni-Wide" M-25 was a long time in development.  One after another, the elaborately hand-crafted prototypes were wired up, checked out and sent back to the bench for refinements.  /soecs became increasingly "excellent," but that wasn't enough.  The basic circuitry effectively fought distortion and efficiently resisted breakdown when operated at full power over long periods of time.  And problems like phase distortion and unwanted high impedance were solved, along with the ever-present conflict between the cost of a component and its real value to the overall performance of amplifier.

"But our engineers were trying for something far beyond mere "excellence."  The researching, re-designing and rfining went on until they found it in the test model they finally approved for actual production.  They found it in the novel Automatic Class AB Switching feature which ensures a wider dynamic range during the reproduction of music.  They found it in the symmetrical mono-times-two const5ruction which eliminates crosstalk, in the DC configuration which removes capacitors from the NFB loop to end phase distortion, and in the high stability and linearity of the extra-large power supply..."

The M-25 was rated at 120 watts continuous, per channel with both channels driven into 8 ohms from 5 Hz to 30,000 Hz, with no more than 0.01% total harmonic distortion.  In fact, this amp could reproduce ultrasonic frequencies up to 80 kHz with only 0.05% THD!  Why have such capability beyond the range of human hearing?  Pioneer stated: "It is essential to the high quality of signals within the hearable range that those beyond that range are handled cleanly in order to avoid harmonic distortion and other harful irregularities.  And this the M-25 does with impressive accuracy, at full power."

To do this, Pioneer employed newly-developed Ring Emitter Transistors (RETs) instead of Field Effect Transistors (FETs).  The RETs had better high frequency characteristics. It employed all the high-grade semiconductors as those used in the M-22, and the same 22,000 microfarad power capacitors.  It naturally had higher output power supplies. 

Overall dimensions of the M-25 were identical  to the M-22, but the M-25 was heavier at 51 pounds, 13 ounces.  M.S.R.P. was $1200.00.