My 303 is one of the last, according to hifi dave who sold hundreds to a large chain of record shops and has, of course, been sensitively "updated" to high heavan by the previous owner. left to idle with no speaker load, it gets gently warm on the heatsinks, so may be set high as it is for all I know.
An old-time work colleague is the son of a once famous pop/country star who had a passion for yodelling
- looked like his Dad too.. When Dad emigrated back to Oz, he left behind his Quad 33/303 and Tannoy Lancasters (I think they were). The speakers had been exposed to high humidity and the voice coil gaps had corrosion in them, needing a complete and expensive rebuild. These were sold twenty years ago for £500 but the amp was retained. It sounded truly awful and exactly as you described above - gritty, noisy and generally unpleasant. Our engineer re-capped a lot of it and re-set up the amp, but although loads better after, it never really sang like I've heard other 33/303's do.
What got me back into 33/303's was the trading in of a lovely set, totally original and sweet as a nut with air and space. Only a small scale sound and frequency-field showed the age of the design as standard. Great that both items can be sonically opened out and allowed to fly
Quad themselves shot us in the feet by recommending QED style passive volume controls for their older power amps -
I got very cross with one of their engineers once as I lost a sale of a 34 because of this.