Low Level Listening and Distortion

Interesting. That’s pretty much what I’m hearing. Do you think my theory about distortion could be the issue? Distortion may cause/trick our minds into thinking depth, three dimensionality, wide soundstage, etc. so the lack of distortion of these newer class D amps leads to the opposite? I will say the AGD amps I heard didn’t sound like this but we know aural memory is rubbish so I could be and probably am wrong. I’ll have to try and hear the AGD again.

Most amplifiers reduce distortion as volume goes up.  Take a look at the distortion vs. output of a linear amp (figure 5) below.

Is it the job of a preamp to be sure the amp sounds the same at all levels or is this just impossible to make the sound that linear?

It's main job is to act as a buffer with variable gain.  The buffer part is that the source doesn't see a variable resistance.  Tubes often lack an output buffer though so the preamp itself may be subject to volume control variances.  The preamp can't compensate for problems in the amp though. 

The difference you are hearing may be due to speaker impedance / amp interaction.  It's possible one amp is responding more to the speaker load than the other.  If that depresses the midrange, at low volumes that may be helpful.

https://www.stereophile.com/content/luxman-l-509x-integrated-amplifier-measurements

Most amplifiers reduce distortion as volume goes up.

Mostly this. Your loudspeakers have way more distortion. There is however a volume level where your system will sound the most linear. That is where you should listen most of the time. 

In other words, a "clean" sounding amp (think stereotypical class D)

'Stereotypical class D'?? IME class D amps vary more in sound from one to another than all tube amps.

It sounds like you are talking about that 'first watt', which has to be musical for the amplifier to have a satisfying portrayal.

Thanks for the response @atmasphere. They may vary in sound but their reputation is a fairly sterile, clean sound as opposed to a tube amp which has a reputation for distortion, no? Ralph, you were who I was thinking of when I started this thread.  I believe in the past you said you like to be able to listen to music at louder volumes and it not sound too loud.  If I'm mistaken, please let me know.  Is it possible an amp that distorts at lower levels would be preferred to one that distorts at higher volume levels for someone that listens at 60 or 65 dB mostly? And, inversely, someone who listens at louder levels would dislike the same amp because once it's turned up it sounds "too loud"? Maybe I'm completely off base as my electrical and amplifier design knowledge is pretty close to zero.