Recommendation for a highly resolving amp

Your equipment is great. It’s the room that is the problem. Take your gear to your friend’s room and then see how it sounds. Or, spend and spend on equipment to no avail. 

I have owned spectral for since the mid 90’s. I don’t know much about your quad but if they are anything like Maggie’s you need at least 250 wpc to Get the panels moving.  
Spectral had or has a universal amplifier that can be driven by any solid state device, so you can use a dac or any other preamp.  The resolution is going to be limited by a tube preamp so I don’t suggest this.
as to spectral. I love their products but they are down to perhaps 3 dealers.  I sold my products through sky fi and at 20 years all passed.  I have seen threads concerning waning repair and customer support.   That is why I chose a different brand when revamping my two channel 

Symphonic Line Kraft, if at all you change the amp for power and resolution

My friends room is significantly larger than mine, so the comparison is really apples and oranges. I can't do anything about the tiny size of my room. Since I do not own a Roon, I don't know how to get the room analyzed. I am using Audiosensibility cables, which are considered one of the best reasonably priced cables.

I am also realizing that the needle never even goes close to the midline, which marks the class A limit of 40W. So, even though ESL are considered power hungry and my M545 is 86db, I think I could get by with around 100 W.

I would try the Sim audio 860 V 2