The Advent 300 Receiver is one of those highly venerated components of solid state audio gear. Advent was a master of the understated advertising campaign for their speakers and they had an understated thing going on with the electronics gear too.
I'll reference the excellent site of David Reaton here as he knows this receiver VERY well and provides tons of information for the 300 for those just interested, and the DIY refurbisher. Plus for those not comfortable around a soldering iron and schematic he will rebuild them. After all it was built in c. 1976 so might need some work by now.
I have one of these receivers and admit I won't keep it much longer as it is going on Craigslist. Mine probably does need work, as perhaps all the electrolytic capacitors replaced. This is pretty normal for gear of this age as they dry out and lose their capacity to...well, capacitate!
I did clean it up a bit and replace the main power supply caps, but have decided I am not going further with so will try to unload it. I also had to customize the main tuning knob for it as the one I have was missing that part when I got it.
These are very well liked for the phono and preamp sections, but I don't need it for that and have just too many pieces of gear so it has to hit the road. It does work in it's current state but caveat emptor.