Recommendations for an Amp/Preamp combo excellent musically and very reliable

I have a similar set up with 3.7i's and REL subs.  I bought used Pass XA 60.8 amps about 5 years ago, leave them on 24/7 and love the sound.  I have had 3 Allnic pre-amps and the Allnic house sound includes a strong holographic element.  The L8000 DHT is sublime with the 3.7i's.  Currently using the DHT pre-amp from a Lamipzator Goldengate DAC.  There is something magical with DHT's and 3.7i's.

There is a point at higher volume levels where the sound changes and loses its liquid character.  It is at a point of high SPL where I would only be listening to a single track or part of a track and not a playlist.  I am not sure if it is the amp running out of juice or it is a room effect/interaction.  I have a large room but one wall is mostly glass doors and windows.

Agree with @atmasphere .  Tube pre and a reputable class D amp. 

I have driven Apogees and Magies with an Aragon 4004 MKII for 30 years along with the Aragon 24k Pre.  Dan D'Agostino (Krell) designed them as Dual Mono that can deliver 200w @8ohms, 400w @4ohms and about 600w into 2ohms.  They can deliver all of the current they are handed.  Absolutely silent background.  I know they are old, but they are really special without having to lay out thousands of dollars.

Power amp: MC² Audio S800

Preamp: ATC SCA2

I use the S800 power amp in my own system, two of them in an active configuration, and though I don't use the SCA2 preamp myself (my DAC has a great preamp section and a digitally controlled, analogue volume control) I know it extremely well from a friend's setup with the exact same S800 power amps. It's an excellent combo through and through.

Consider the CANOR brand from Slovakia. You can mix and match SS and Tube models to suit your taste. Super reliable and tubes are very well matched at the factory.