System Plateau - Ideas?

I have an all Linn Akurate/Sondek/Keilidh Aktiv system in my living room, but spend more time in my basement office with a Knight Kit KA-40 32W tube amp (which my Dad originally built in 1960), Smaller Advents, Apogee Duet, Apple Music.

Smaller Advents were my first hi-fi purchase in 1973, along with a Dynaco SCA-80Q kit and AR XB with Stanton 681EEE. Most would scoff...

The Knight/Advent system is so much fun to listen to, involving, intimate, subtle.

My advice to all: enjoy the music, forget the equipment.

@dotchaos
@decooney I won't let go of the FirstWatt, also won't let go of the Line Magnetic (at least until I can afford the 845 premium). I did take the LM as far as I could with tube selection, but didn't get to playing around with caps at all.

With a push from a few local mentors who've been doing this for 60+ years, they led me down a path that started with tube rolling and ended with high-end coupling caps on my last three tube amp setups, fwiw.  

op

as miller appropriately called out...  stating what you are trying to improve with the right words to convey your meaning precisely is all important here... as you can see from the fairly scatter shot replies you have received thus far

dotchaos
Not completely grasping your idea of "edge" or "fullness", Nelson Pass built the FW aleph with a reduction of negative feed back but makes the claim of comparable distortion to his previous amps. Ok...so maybe not with what your combo provides even though he places this amp in second harmonics.
I believe this is what your ears are missing in what you describe as "fullness"???
This is basically a design that relies on phase distortions to produce variations in sound response.
Many people mix a tube pre with an ss amp as you have to attempt to get this "perfect balance" of "musical weight" to "crispness" of sound and for this to them may say "fullness" in capturing highs and lows to their preference.
Your complaint is "age old" when people move from tube to ss. It basically comes down to what you are used to tied in with what you have learned as your "preference sound".
Some people like that "tube bloom" (not to the extent of "bloat" however) tube amps may have as your LM more than likely displayed. It is what gives "weight" to music sound production with some compromise to "clarity" in upper registers.
So I am not quite sure that when you say "fullness" you are meaning "musical heft or weight" or if your description is trying to describe what you consider a perfect blend in all harmonic registers.
At any rate, I’ve tried to touch on all parameters but may have failed miserably for you.

It’s very hard to describe what you want from an upgrade when the margins are small.

i had a significant SQ up lift moving to SP5 Morrow Speaker cables and the cables i had previously were already pretty good. 

i believe that’s where you will get the best ROI.