I'm a past-owner of QuadIIs and a current 34/405 user.
Personally, I can see no reason to go back.
I dare say others will disagree.
Speaking personally as an engineer, an amplifier is a 'bit of kit' that does a job, a black box.
I want the signal to go in one end, and come out the other, with suitably more 'ooommph' but with as little deviation from the input signal as possible.
I read somewhere that Walker saw no point in changing from valve to transistor amps until there was good reason to do so.
I'm pretty sure that Quad were the last of what I call the 'big three' (The others, ofcourse, being Leak and Radford) to put transistor amps into production.
In my opinion, and my own experience, nothing else quite comes close to a Quad405.
No doubt, you have read Gordon J. King's report, where he was struggling to measure distortion with all of the (at that time) equipment available at his disposal.
http://quad405.com/testreports.pdf
OK, so you couldn't get an AP in 1976, but I think it proves a point.
You will also read of reliability problems, well, I've not had any, maybe I've been lucky.
The 44 is a useful bit of kit, slightly more versatile than the 34, which meets my need suitably (turntable and CD player) Circuitry inside is very similar.
It has long been my belief that Peter Walker is the most gifted audio engineer, (with Peter Baxandall a close second.)
I can't comment on the FM4, I'm afraid!