Hello
@GusGuT and All, late but long reply, apologies...
Regarding the HK680 it's definitely worth to read Dejan Veselinovic's review at tnt-audio. Written by someone who built custom high-end transistor amps for many years, his good words have some ground to me.
My experiences following are all with used units and subjective, take them with a grain of salt.
Consider the fact that any used amp nearing 20-30 years of age should be thoroughly checked before use, even more the H/K-s which are direct coupled without protection for the failure of the output stage, which can make significant damage to your precious speakers.
With all that said i have 4 HK680s and a bunch of other H/K amps waiting for restoration, i just love the way they sound.
If you are willing to diy, check out Trevor's bench at youtoube, he has a lot of good H/K repair videos, also Xraytonyb for a detailed restoration of the PM665.
No experiences with the 6500 but you simply cannot find a bad word about this amp on the net (considering its price ofc) so it should be a safe bet.
If you can, avoid the HK630, HK650, HK690 which use mediocre opamps and have inferior prestages. Their power amp section is however still discrete and excellent for the price.
The HK680 was my first decent amp, source was a Chord Mojo, speakers were Elac EL160 IIs (old totl 4 Ohm floorstanders). I was stunned by the performance this system gave. The Elacs were extremely fast and let the 680 shine. At that time i had a H/K AVR10 and a Yamaha RX-V359. The AVR10 did a fair job for the 20 or 25 Euros i've bought it, but was nowhere near the 680 in each and every aspect. The Yammie was loud and more detailed, somehow lacking bass and cleanliness (or black background) and sounding unbearably artificial, tiring and uninvolving compared to the 680. Perhaps it's not a fair comparison, i'd still go for the AVR10 over the RX-V359 though.
The 680 was made in Korea and sounded subjectively better (more meat and excitement, midrange, immediacy) than the Japanese made ones i later acquired, which are still very good. Perhaps the age, the caps, who knows?
The PM665 sounded ok but the highest range was missing, i suspect the caps and carbon comp resistors were way out of tolerance. According to Xraytonyb, these are hard to service and the pcbs and tracks are brittle.
Later on the speakers were changed to Mission 767s and although the 680 drives them to a beautiful, clean and powerful sound, it slightly lacks the juice and grip to really make them sing.
Among the H/K AVRs the AVR8500 (= AVR8000 in the USA) is something special (...), best used with direct inputs (everything is bypassed but the volume control) or the source fed directly into the power amps (no preamp, no volume c).
Anyone considering the HK680, just give it a try, have it checked by a tech and be amazed.
