Tube Pre-Amp, vs Solid State with Tone Controls/EQ to take edge off the top end?

The Loki certainly seems well worth trying. One downside, though, might result from the fact that it only provides unbalanced inputs and outputs, and presumably has an unbalanced internal signal path, while I believe both of your Pass components are fully balanced.

In any event, if the Loki proves to not be to your liking, and you would be willing to consider spending several thousand dollars on a remarkably versatile and transparent solution, you may want to consider a DEQX, www.deqx.com

It accomplishes equalization via sophisticated digital signal processing, without the traditional side-effects and limited adjustability of analog equalization, and can do a whole lot more than just serving as an equalizer.

Most models provide digital inputs and outputs as well as balanced and unbalanced analog inputs and outputs, as well as the ability to select between multiple sources, so inserting it into the signal path before or after your preamp are both possibilities that could be considered. Some models can also be used as a preamp, which is what I do with my DEQX HDP-5.

For a lot more commentary by DEQX users see this thread:

https://forum.audiogon.com/discussions/is-deqx-a-game-changer

Good luck. Regards,
-- Al

I have Thiels and I used a BAT pre with SS amp to "take the edge" off the highs. A bit of tube rolling dialed it right in.

the BAT is fully balanced too. 

@last_lemming What BAT combo are you using?

BAT has always interested me.  

Agreeing w Elizabeth on cleaning power first. You have a good pre. All a softer tube pre could do for you is roll off the top which you’ll soon find boring. A neutral tube pre will leave you in the same place.
 I have found the latest generation of AQ power products to accomplish what you are looking for without rolling the highs off. In fact the Hurricane power cord is the first high end cord that I can say provided fundamental benefits along the lines you are looking for. I am somewhat sceptical on power cords but not this one. FWIW

Well, I am not a believer in power cleaning etc.  Many MFG want you to plug straight into the wall, I have a dedicated 20amp outlet etc.  

I appreciate many do.  I've used these electronics with a few speakers.  Never an issue with brightness.  It's the speakers, I am aware of that so just looking to tweak it a shade.

I don't want to get off track, but felt I should at least respond.