Hi Dave,
Quad the family owned British company effectively ended with the 6 series in the 90s.
Last of the line were the 66 pre, tuner and CD, plus the 606 power amp.
Those are all closely related to the earlier 4 series in terms of circuit though with refinements and things such as remote control.
Later kit veers off in two directions. The big power amplifiers such as the 707 and 909 retain current dumping and are very similar internally to a 606.
The slimline stuff has no real connection to earlier Quad circuits and IIRC the new owners used well known industry designers to produce that kit.
Modern valve Quad is of course designed by TdP.
For me, real Quad ends with the 6 series electronics and the ESL63 loudspeaker.
True PJW period ended with the 4 series as he effectively retired shortly after and left the company to be run by his son Ross.
To answer your question, the modern current dumpers are excellent and technically better than the 405. Better drive into low impedance and lower noise floor.
One often overlooked amp is the small 306.
This takes the current dumping circuit from the 405, dispenses with the op amp input stage, moves to near dual mono design running from twin secondaries and moves the current limiting from the amplifier circuit and reverts to a simple current/thermal trip in the mains line. If 50wpc is enough and your speakers don't dwell below 4 ohms it is a very fine little amp.
Nice with Tannoys
I have 303, 306 and 405 here (all serviced) so if you ever want try the 405 you are most welcome to borrow it.