System Plateau - Ideas?

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.

If the bass in the room has not been “managed” then everything else is a mere compromise as it will smear upstream frequencies and you chase an elusive demon. 

@dotchaos
"edge". That wasn’t the right word at all. I think what I’m missing is the fullness/roundness of the Line Magnetic. The slightly "thinner" sound of the current set up leaves me wanting a bit.

@audition__audio do you have a preferred brand you’d recommend, for cables? I’m using ICs from an Etsy outfit called Amplifier Surgery, Blue Jean speaker cables. All very low-budget. Forgot to mention that I’ve been looking to upgrade the turntable, as well. Haven’t seen a clear next choice, there (many options).

@dotchao
Looking back over a few of your comments and others. A few touched on this already, and I’ll add a few comments in support of this angle.

Yes, "cables" and and interconnects specifically may very well be one area to re-focus on as a next step with the "current (J2) setup" as you referred to in your last update. Yes, your LM518ia with 845s in Triode mode will have a lush and more rounded off sound with more fullness. You can tone-tune the J2 setup a bit with different ICs, for sure.

To save $ and prevent costly do-overs, you could reach out to TheCableCo and take advantage of their cable Loaner Program. Or, find a helpful local dealer who’ll let you take home 3-4 pairs of different "well used" interconnects to try for a few weeks. Ask for warm IC cables...

FYI, I’ve re-sold multiple pairs of older legacy high end Cardas ICs to Nelson amp owners trying to smooth out or fill-out the sound a bit more due to the leanness you referred to in your current setup. Root cause can be speakers/matching, but you can cheat some by trying and rolling some different interconnects with your J2 (IF) you plan to stay with your current SS amp and speakers. Will say an 845/Triode amp is pretty unique and hard to duplicate compared to most SS amps, yet the J2 is the one I'd pick if I were doing lower power FirstWatt SS.  Good luck.