I've got a pair of 442's - which were the little/big brother /successor to the 551's - although they're now in storage.
The bass on my 442's was stunning - according to the little leaflet I've got here describing the 551, 442 and 662, 330mm/13" bass driver in a sealed box (which to my ears was always superior to a vented box. Your 551 is specified as 290mm (11.4") and is vented box so YMMV.
Over the years I found the treble harsh, especially (I suspect) with the loss of the little perforated covers over the tweeters, and that's the major reason why they were retired.
Stereo imaging struggles because of the design of the front baffle. The front baffle is rebated slightly behind the front edges of the cabinet, which I don't believe helps treble dispersion, and in any case imaging (well, almost everything) was dramatically improved (to my ears) by removing the grilles - thick pieces of chipboard that made the rebating way worse.
If I was looking for matching equipment, i don't think I'd be looking at the Quad 405. (but then I don't think I'd ever look at the Quad 405). Although I've never heard one, the Quad was the amp that started the pain of DBTs back in the 70's. You can read about it at
Stereophile
Over 10 years ago, for example, I failed to distinguish a Quad 405 from a Naim NAP250 or a TVA tube amplifier in such a blind test organized by Martin Colloms (footnote 2). Convinced by these results of the validity in the Consumer Reports philosophy, I consequently sold my exotic and expensive Lecson power amplifier with which I had been very happy and bought a much cheaper Quad 405ââ'¬â€the biggest mistake of my audiophile career!
I think you need a smooth amplifier for these speakers - I suspect the 405 won't fit the bill - or new tweeters. As it happens (completely unconnected with the Stereophile article) I used to have (okay, I still have, but in need of repair) a Lecson AC1 AP3 combination. The lecson power amps were beautifully smooth and effortless. back in the 70's virtually every amp was rolled off at 20kHz, so I'd start by trying a something with a softer 70's sound - a good old NAD would get you started. Where you go from there is up to you. If I think back, I think the point at which I started disliking the treble was when I purchased a Meridian 101MC as a moving coil preamp which replaced my Lecson AC1, but was never a nice treble.
At the back of my mind I want to rebuild the 442's - even if only as a pair of bass units, but I suspect this will never happen. And why did I have a pair of 442's? because I won them in a competition in a hifi mag - HiFi for Pleasure IIRC.
PM me with your address and I'll send a photcopy of the leaflet.
