I'm going to post a bit of a contrary opinion - I had a mid-70's Kenwood integrated amp - I think the model number was something like KA-7100, or thereabouts - which was a very nice design, IMO.
It was rated at 60 wpc (RMS), with 0.02% THD (20-20k, if not a larger frequency range - see below).
This unit was built like a tank:
- separate power supplies for each channel (dual mono construction)
- no capacitors in the signal path - as a result the frequency response was rated as 0 Hz. to 100kHz. The amp would actually amplify DC, and so they had a switchable "subsonic" filter that was there in case you needed DC blocking due to a noise or grounding issue.
- zero phase shift - the manual included Lissajous plots of the output versus the input waveform at several frequencies - these were a straight line (Lissajous plots are done by putting the output waveform on one scope probe, the input on the other, and putting the scope in X/Y plot mode. A straight line indicates that the output is phase-aligned with the input.)
- a very nice resistor-ladder stepped attenuator volume control (i.e., not a potentiometer), with a beautifally-machined and polished (and large) volume knob.
- In addition to the balance, treble, and bass controls, and the afore-mentioned subsonic filter, there was an additional high-frequency switchable filter, with a 3 dB point somewhere around 10kHz, plus a three position loudness switch (bass boost), and a -20 dB attenuator switch, in case you wanted either more volume-control resolution at low volume levels, or just wanted to flick a switch to answer the phone. The tone controls had a defeat switch, as well.
The sound was very clean - and the 60 wpc was plenty to drive the speakers I had at the time, a pair of Large Advents (not too efficient!)
(As an aside, I once had this amp driving my Advents in one room, and a pair of Altec Lansing Voice of the Theaters in another room, for a party. In order to hear anything out of the Advents, the volume in the room with the VOT's was almost unbearable!)
It may be that this particular line of Kenwood integrated amps was done better than some of their other products - I don't think they used the same designs in their receivers. But I would have stacked my amp up against a Marantz amp from the same period. (Although definitely not in terms of collector value - those early Marantz amps from the 50's and 60's are indeed classics!)