how well Quad303 amp mates with B&W dm303's ?

The B&W DM303s are a very nice and easy to drive speakers. I bought them for my Dad and when I had them in my system they sounded great. No issues at all. I drove them with a rebuilt Hafler DH-220 as well as a rebuilt Sumo Nine and the music was very enjoyable. I would say if the costs aren't too much just get the Hafler 110 and DH-220 refurbed. Many kits are available to bring them up to snuff.

Ideally they could use more power, although the DM303s are not super picky. I use them with an NAD 314. They sound fine with my limited expectations but sound fuller with my 75 watt amp. If the Quad's output dumps at lower impedance, that effect is probably the opposite of how these speakers were designed. I have no experience with quad amps, so that is my speculation. Good speakers.

I use to be a Quad dealer in the days of the 303 and it was a good amp in its day. Output fell off at lower impedance's; I didn't want to sell one to someone who liked the sound on his AR 3s as they were 4 ohms and the 303 would only put out something like 16 watts at 4 ohms. He bought something else and later told me he wished he had gotten the 303 despite the limited power. I was a Hafler dealer also and the 100 and 220 were good units. You might want to consider having them renewed with new resistors and/or caps. I never had the 303 and 220 at the same time so can't say how they would sound on same speakers. I was a B&W dealer also but before the DM303's were out. B&W make good speakers; I had a pair of their first speakers, P2Hs; which used an ionic tweeter; I still have a pair of Matrix 805s. The newer Quads are also a good value used in my mind; I haven't been a dealer for any product I mentioned for at least 20 years so I have no interest in their sales.