Lots of knobs might cause more repair chance

Not necessarily true, but if you buy any piece of vintage equipment, you should have it thoroughly checked by a qualified technician which can add several hundred dollars to your ownership costs.

I don't think, I just know it's good gear and will always be good to use and will always be worth to service and it has published service manuals with schematics.
If you go for newer components, they mostly compromise in built quality, functionality and no service manuals or schematics. Instead of going to local tech around the corner, you will have to send it to 'authorized' repair facilities or factory and once such factory release a new model your old model will be forgotten and so is the money you've spend will turn to a few bucks.
Stick to this unit as it will never loose value. After all, it's not too many knobs compared to other vintage units.

Thanks for your kind reply.  The problem is the source of the 2nd hand Amplifier. Most of them were not sold by the formal distributor, so it is impossible to get after service from the authorized technicians served in the distributor.   Also, the old equip. might run out of parts either, that means you might unable to get is fixed.  Even though some good old gear were using good quality parts, but after 20 years or more usage, there is higher possibility of repair requirement.  You have no way but to find the service engineer around the  corner.

I repair older vintage units and get great parts from digikey.com

Digikey automatically brings you replacement versions to the original circuit element with same or better parameters. Even besides that, the parameters of older semiconductors are not problem to acquire and so is finding one over the major electronic parts distributor isn't a biggie at all. No need for service engineer around corner.

I mostly deal with Pioneer SX series solid state receivers, Sansui G-series receivers and AU amps and had no issue finding semiconductors and other circuit elements. More often than not rebuilding the channel with new transistors substantially improves sound. I had one unit where I overhauled only one channel that sounded a lot better than healthy channel that I also had to rebuild with same components.

ebm double negation product in math gives positive magnitude.