Well, I for one think that the M9994 is worth every bit the difference in cost compared to the M9990. I have several examples of each and have worked extensively on both of them. The M9994 is definitely much better sounding, and WAY more powerful. Specs from service manuals: M9990 = 2.7wpc output power, M9994 = 7.0wpc output power. The M9994 also has a phono and auxillary line in function. The M9990 is sorely lacking this important feature. Analog tuning meter on the M9994 for signal strength sensitivity. LED tuning indicator for M9990. M9990 does have clock/timer, which M9994 does not but instead relies on standard sleep feature.
Electrical design: M9990 only has adjustments for Playback gain, Recording gain and meters.
M9994 has adjustments for Playback gain, Recording gain, meters, ALC and Bias adjustment. NO BIAS ADJUSTMENT is prescribed in the service manual for the M9990!!!
Also, this could become a very important adjustment: Tape playback speed. M9994 is adjustable to compensate. M9990 has no adjustment for tape speed.
Bottom Line: Both are good boxes, M9990 is plentiful and fairly cheap. M9994 is much rarer and very pricey but in my opinion, worth every penny!